Display device and method for manufacturing the same

ABSTRACT

A first organic resin layer is formed over a first substrate; a first insulating film is formed over the first organic resin layer; a first element layer is formed over the first insulating film; a second organic resin layer is formed over a second substrate; a second insulating film is formed over the second organic resin layer; a second element layer is formed over the second insulating film; the first substrate and the second substrate are bonded; a first separation step in which adhesion between the first organic resin layer and the first substrate is reduced; the first organic resin layer and a first flexible substrate are bonded with a first bonding layer; a second separation step in which adhesion between the second organic resin layer and the second substrate is reduced; and the second organic resin layer and a second flexible substrate are bonded with a second bonding layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/872,806, filed May 12, 2020, now allowed, which is a continuation ofU.S. application Ser. No. 16/856,139, filed Apr. 23, 2020, now U.S. Pat.No. 10,872,947, which is a continuation of U.S. application Ser. No.16/204,102, filed Nov. 29, 2018, now U.S. Pat. No. 10,854,697, which isa continuation of U.S. application Ser. No. 15/894,117, filed Feb. 12,2018, now U.S. Pat. No. 10,355,067, which is a continuation of U.S.application Ser. No. 15/416,166, filed Jan. 26, 2017, now U.S. Pat. No.10,312,315, which is a continuation of U.S. application Ser. No.15/145,246, filed May 3, 2016, now U.S. Pat. No. 9,559,316, which is acontinuation of U.S. application Ser. No. 14/553,251, filed Nov. 25,2014, now U.S. Pat. No. 9,437,831, which claims the benefit of foreignpriority applications filed in Japan as Serial No. 2014-047348 on Mar.11, 2014, Serial No. 2013-272176 on Dec. 27, 2013, Serial No.2013-256872 on Dec. 12, 2013, and Serial No. 2013-249631 on Dec. 2,2013, all of which are incorporated by reference.

TECHNICAL FIELD

One embodiment of the present invention relates to a semiconductordevice including an oxide semiconductor, a display device including thesemiconductor device, and a manufacturing method thereof.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of one embodiment of theinvention disclosed in this specification and the like relates to anobject, a method, or a manufacturing method. In addition, one embodimentof the present invention relates to a process, a machine, manufacture,or a composition of matter. Specifically, examples of the technicalfield of one embodiment of the present invention disclosed in thisspecification include a semiconductor device, a display device, a liquidcrystal display device, a light-emitting device, a lighting device, apower storage device, a storage device, a method for driving any ofthem, and a method for manufacturing any of them.

In this specification and the like, a semiconductor device generallymeans a device that can function by utilizing semiconductorcharacteristics. A transistor and a semiconductor circuit areembodiments of semiconductor devices. In some cases, a storage device, adisplay device, or an electronic device includes a semiconductor device.

BACKGROUND ART

A technique by which a transistor is formed using a semiconductor filmformed over a substrate having an insulating surface has been attractingattention. The transistor is applied to a wide range of electronicdevices such as an integrated circuit (IC) or an image display device(display device). A silicon-based semiconductor material is widely knownas a material for a semiconductor thin film applicable to a transistor.As another material, an oxide semiconductor has been attractingattention.

For example, a transistor whose active layer includes an amorphous oxidesemiconductor containing indium (In), gallium (Ga), and zinc (Zn) isdisclosed in Patent Document 1.

For a display device, it is required to improve the flexibility orimpact resistance besides a reduction in the thickness and weight. Forexample, Patent Document 2 discloses a flexible active matrixlight-emitting device in which an organic EL element and a transistorserving as a switching element are provided over a film substrate.

REFERENCE [Patent Document]

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528-   [Patent Document 2] Japanese Published Patent Application No.    2003-174153

DISCLOSURE OF INVENTION

In a process of manufacturing a flexible display device, a minutedefective portion that is not a problematic part when a rigid substrateis used may increase, which may reduce the manufacturing yield.Furthermore, the defective portion may increase due to warp or bendafter the display device is completed, which may reduce the displayquality and reliability.

Thus, in a method for manufacturing a flexible display device, anappropriate combination of materials or processing method is desirablyused so as not to generate a minute defective part in a product that isin a manufacturing process.

It is an object of one embodiment of the present invention to provide adisplay device having high display quality. Another object is to providea display device with high reliability. Another object is to provide anovel display device. Furthermore, another object is to provide a novelsemiconductor device or the like. Another object is to provide a methodfor manufacturing the display device.

Note that the descriptions of these objects do not disturb the existenceof other objects. In one embodiment of the present invention, there isno need to achieve all the objects. Other objects will be apparent fromand can be derived from the description of the specification, thedrawings, the claims, and the like.

Means for Solving the Problems

One embodiment of the present invention relates to a flexible displaydevice including a transistor using an oxide semiconductor layer and amethod for manufacturing the flexible display device.

One embodiment of the present invention is a method for manufacturing adisplay device that includes a first element layer and a second elementlayer one of which includes a pixel portion and a circuit portion andthe other of which includes a coloring layer and a light-blocking layer.The pixel portion includes a first transistor including an oxidesemiconductor layer and a display element, and the circuit portionincludes a second transistor including an oxide semiconductor layer. Themethod includes the following steps: a step of forming a first organicresin layer over a first substrate; a step of forming a first insulatingfilm over the first organic resin layer; a step of forming the firstelement layer over the first insulating film; a step for forming asecond organic resin layer over a second substrate; a step of forming asecond insulating film over the second organic resin layer; a step offorming the second element layer over the second insulating film; a stepof bonding the first substrate and the second substrate so as to enclosethe first element layer and the second element layer; a step of reducingthe adhesion between the first organic resin layer and the firstsubstrate, thereby separating the first substrate (a first separationstep); a step of bonding the first organic resin layer and a firstflexible substrate with a first bonding layer; a step of reducing theadhesion between the second organic resin layer and the secondsubstrate, thereby separating the second substrate (a second separationstep); and a step of bonding the second organic resin layer and a secondflexible substrate with a second bonding layer.

In this specification, ordinal numbers such as “first”, “second”, andthe like are used in order to avoid confusion among components, and theterms do not limit the components numerically.

The first organic resin layer and the second organic resin layer can beformed using a material selected from an epoxy resin, an acrylic resin,a polyimide resin, a polyamide resin, or a polyamide-imide resin.

It is preferable to perform the irradiation with a linear excimer laserto reduce the adhesion between the first organic resin layer and thefirst substrate and the adhesion between the second organic resin layerand the second substrate.

The above excimer laser is preferably a laser light obtained bysynthesizing lasers outputted from a plurality of oscillators.

The second separation step is preferably performed in such a manner thatthe first flexible substrate is in contact with a curved surface of aroller.

Each of the first insulating film and the second insulating filmpreferably include a silicon oxide film, a silicon oxynitride film, asilicon nitride film, or a silicon nitride oxide film.

For the oxide semiconductor layer, an In-M-Zn oxide (M is Al, Ti, Ga, Y,Zr, La, Ce, Nd, Sn, or Hf) can be used. The oxide semiconductor layerpreferably includes a c-axis aligned crystal.

As the display element, an organic EL element can be used.

Another embodiment of the present invention is a display deviceincluding the following components: a first flexible substrate; a firstbonding layer; a first organic resin layer; a first insulating film; afirst element layer that includes a pixel portion including a firsttransistor including an oxide semiconductor layer and a display elementand that includes a circuit portion including a second transistor; asecond element layer including a coloring layer and a light-blockinglayer; a second insulating film; a second organic resin layer; a secondbonding layer; and a second flexible substrate, where the abovecomponents are stacked in this order.

The oxide semiconductor layer included in the first transistor may be asingle layer, and the oxide semiconductor layer included in the secondtransistor may be a multilayer.

The oxide semiconductor layer included in the first transistorpreferably has the same composition as a layer in contact with a gateinsulating film of the second transistor including an oxidesemiconductor layer.

According to one embodiment of the present invention, a display devicewith favorable display quality can be provided. Alternatively, accordingto one embodiment of the present invention, a highly reliable displaydevice can be provided. Alternatively, according to one embodiment ofthe present invention, a novel display device or the like can beprovided. Alternatively, according to one embodiment of the presentinvention, a method for manufacturing the display device can beprovided. Alternatively, according to one embodiment of the presentinvention, a method for manufacturing the display device with a highmanufacturing yield can be provided.

Note that the description of these effects does not disturb theexistence of other effects. One embodiment of the present invention doesnot necessarily achieve all the effects listed above. Other effects willbe apparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a top view illustrating a display device.

FIG. 2 is a cross-sectional view illustrating a display device.

FIG. 3 is a cross-sectional view illustrating a display device.

FIGS. 4A to 4D are cross-sectional views illustrating a method formanufacturing a display device.

FIGS. 5A to 5C are cross-sectional views illustrating a method formanufacturing a display device.

FIG. 6 illustrates an example of a processing apparatus using an excimerlaser.

FIGS. 7A to 7E illustrate an example of a separation apparatus.

FIGS. 8A to 8C illustrate an example of a separation apparatus.

FIGS. 9A to 9C illustrate an example of a separation apparatus.

FIGS. 10A to 10E illustrate an example of a separation apparatus.

FIGS. 11A to 11C illustrate an example of a separation apparatus.

FIGS. 12A to 12C illustrate an example of a separation apparatus.

FIGS. 13A to 13C illustrate an example of a separation apparatus.

FIGS. 14A and 14B illustrate an example of a separation apparatus.

FIGS. 15A1, 15A2, 15B1, 15B2, 15C1, and 15C2 illustrate an example of aseparation apparatus.

FIGS. 16A1, 16A2, 16B1, 16B2, 16C1, and 16C2 illustrate an example of aseparation apparatus.

FIGS. 17A to 17C are a block diagram and circuit diagrams showing adisplay device.

FIG. 18 illustrates a display module.

FIGS. 19A and 19B are each a cross-sectional view illustrating atransistor.

FIGS. 20A and 20B are each a cross-sectional view illustrating atransistor.

FIGS. 21A to 21D are Cs-corrected high-resolution TEM images of a crosssection of a CAAC-OS and a cross-sectional schematic view of theCAAC-OS.

FIGS. 22A to 22D are Cs-corrected high-resolution TEM images of a planeof a CAAC-OS.

FIGS. 23A to 23C show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD.

FIGS. 24A to 24D each illustrate an electronic device.

FIGS. 25A and 25B are each a cross-sectional view illustrating atransistor.

FIGS. 26A and 26B are each a cross-sectional view illustrating atransistor.

FIGS. 27A and 27B are each a cross-sectional view illustrating atransistor.

FIGS. 28A and 28B are each a cross-sectional view illustrating atransistor.

FIGS. 29A and 29B are each a cross-sectional view illustrating atransistor.

FIG. 30 is a cross-sectional view illustrating a display device.

FIG. 31 is a cross-sectional view illustrating a display device.

FIGS. 32A to 32D are cross-sectional views illustrating a method formanufacturing a transistor.

FIGS. 33A to 33D are cross-sectional views illustrating a method formanufacturing a transistor.

FIGS. 34A to 34D are cross-sectional views illustrating a method formanufacturing a transistor.

FIGS. 35A and 35B are cross-sectional views each illustrating atransistor.

FIG. 36 shows temperature dependence of resistivity.

FIGS. 37A to 37F are each a cross-sectional view illustrating atransistor.

FIGS. 38A to 38F are each a cross-sectional view illustrating atransistor.

FIGS. 39A to 39E are each a cross-sectional view illustrating atransistor.

FIGS. 40A and 40B show electron diffraction patterns of a CAAC-OS.

FIG. 41 shows a change in crystal parts of In—Ga—Zn oxides induced byelectron irradiation.

FIGS. 42A and 42B are schematic diagrams illustrating deposition modelsof a CAAC-OS layer and an nc-OS layer.

FIGS. 43A to 43C show an InGaZnO₄ crystal and a pellet.

FIGS. 44A to 44D are schematic diagrams illustrating a deposition modelof a CAAC-OS.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail with reference to drawings. Notethat the present invention is not limited to the following descriptionand it will be readily appreciated by those skilled in the art thatmodes and details can be modified in various ways without departing fromthe spirit and the scope of the present invention. Therefore, thepresent invention should not be interpreted as being limited to thedescription of embodiments below. Note that in structures of the presentinvention described below, the same portions or portions having similarfunctions are denoted by the same reference numerals in differentdrawings, and description thereof is not repeated in some cases. It isalso to be noted that the same components are denoted by differenthatching patterns in different drawings, or the hatching patterns areomitted in some cases.

For example, in this specification and the like, an explicit description“X and Y are connected” means that X and Y are electrically connected, Xand Y are functionally connected, and X and Y are directly connected.Accordingly, another element may be provided between elements having aconnection relation illustrated in drawings and texts, withoutlimitation on a predetermined connection relation, for example, theconnection relation illustrated in the drawings and the texts.

Here, X and Y each denote an object (e.g., a device, an element, acircuit, a line, an electrode, a terminal, a conductive film, a layer,or the like).

Examples of the case where X and Y are directly connected include thecase where an element that allows an electrical connection between X andY (e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, and a load) is notconnected between X and Y, and the case where X and Y are connectedwithout the element that allows the electrical connection between X andY provided therebetween.

For example, in the case where X and Y are electrically connected, oneor more elements that enable electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. A switch is controlled to be on or off. Thatis, a switch is conducting or not conducting (is turned on or off) todetermine whether current flows therethrough or not. Alternatively, theswitch has a function of selecting and changing a current path. Notethat the case where X and Y are electrically connected includes the casewhere X and Y are directly connected.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power supply circuit (e.g., a dc-dcconverter, a step-up dc-dc converter, or a step-down dc-dc converter) ora level shifter circuit for changing the potential level of a signal; avoltage source; a current source; a switching circuit; an amplifiercircuit such as a circuit that can increase signal amplitude, the amountof current, or the like, an operational amplifier, a differentialamplifier circuit, a source follower circuit, or a buffer circuit; asignal generation circuit; a memory circuit; and/or a control circuit)can be connected between X and Y. Note that for example, in the casewhere a signal output from X is transmitted to Y even when anothercircuit is interposed between X and Y, X and Y are functionallyconnected. Note that the case where X and Y are functionally connectedincludes the case where X and Y are directly connected and the casewhere X and Y are electrically connected.

Note that in this specification and the like, an explicit description “Xand Y are electrically connected” means that X and Y are electricallyconnected (i.e., the case where X and Y are connected with anotherelement or another circuit provided therebetween), X and Y arefunctionally connected (i.e., the case where X and Y are functionallyconnected with another circuit provided therebetween), and X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween). That is, inthis specification and the like, the explicit description “X and Y areelectrically connected” is the same as the description “X and Y areconnected”.

Note that, for example, the case where a source (or a first terminal orthe like) of a transistor is electrically connected to X through (or notthrough) Z1 and a drain (or a second terminal or the like) of thetransistor is electrically connected to Y through (or not through) Z2,or the case where a source (or a first terminal or the like) of atransistor is directly connected to one part of Z1 and another part ofZ1 is directly connected to X while a drain (or a second terminal or thelike) of the transistor is directly connected to one part of Z2 andanother part of Z2 is directly connected to Y, can be expressed by usingany of the following expressions.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Other examples of the expressions include, “a source (or a firstterminal or the like) of a transistor is electrically connected to Xthrough at least a first connection path, the first connection path doesnot include a second connection path, the second connection path is apath between the source (or the first terminal or the like) of thetransistor and a drain (or a second terminal or the like) of thetransistor, Z1 is on the first connection path, the drain (or the secondterminal or the like) of the transistor is electrically connected to Ythrough at least a third connection path, the third connection path doesnot include the second connection path, and Z2 is on the thirdconnection path”. It is also possible to use the expression “a source(or a first terminal or the like) of a transistor is electricallyconnected to X through at least Z1 on a first connection path, the firstconnection path does not include a second connection path, the secondconnection path includes a connection path through the transistor, adrain (or a second terminal or the like) of the transistor iselectrically connected to Y through at least Z2 on a third connectionpath, and the third connection path does not include the secondconnection path”. Still another example of the expression is “a source(or a first terminal or the like) of a transistor is electricallyconnected to X through at least Z1 on a first electrical path, the firstelectrical path does not include a second electrical path, the secondelectrical path is an electrical path from the source (or the firstterminal or the like) of the transistor to a drain (or a second terminalor the like) of the transistor, the drain (or the second terminal or thelike) of the transistor is electrically connected to Y through at leastZ2 on a third electrical path, the third electrical path does notinclude a fourth electrical path, and the fourth electrical path is anelectrical path from the drain (or the second terminal or the like) ofthe transistor to the source (or the first terminal or the like) of thetransistor”. When the connection path in a circuit structure is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope.

Note that these expressions are examples and there is no limitation onthe expressions. Here, X, Y, Z1, and Z2 each denote an object (e.g., adevice, an element, a circuit, a wiring, an electrode, a terminal, aconductive film, and a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

Note that the terms “film” and “layer” can be interchanged with eachother depending on the case or circumstances. For example, the term“conductive layer” can be changed into the term “conductive film” insome cases. Also, the term “insulating film” can be changed into theterm “insulating layer” in some cases.

Embodiment 1

In this embodiment, a display device that is one embodiment of thepresent invention and a manufacturing method thereof will be describedwith reference to drawings.

The “display device” in this specification means an image display deviceor a light source (including a lighting device). Further, the displaydevice includes any of the following modules in its category: a moduleincluding a connector such as a flexible printed circuit (FPC), or tapecarrier package (TCP); a module including TCP which is provided with aprinted wiring board at the end thereof; and a module including a drivercircuit which is directly mounted on a display element by a chip onglass (COG) method.

The display device that is one embodiment of the present invention hasflexibility (flexible display device). Note that “flexible device” meansthat a device that can be bent or warped. The flexibility can beutilized in an end product in some cases and utilized in a manufacturingprocess in some cases. In the latter case, the end product has noflexibility in some cases.

FIG. 1 is a top view of a display device 300 that is one embodiment ofthe present invention. In FIG. 1, some components are enlarged, reducedin size, made to be visible, or omitted for easy understanding.

The display device 300 includes a pixel portion 302 over a firstflexible substrate 301, a first circuit portion 304 and a second circuitportion 305 configured to drive the pixel portion, a sealant 312provided to surround the pixel portion 302, the first circuit portion304, and the second circuit portion 305, and a second flexible substrate307 provided to face the first flexible substrate 301. Note that asignal line driver circuit (source driver) and a scan line drivercircuit (gate driver) can be used, for example, as the first circuitportion 304 and the second circuit portion 305, respectively.

The first flexible substrate 301 and the second flexible substrate 307are bonded to each other with the sealant 312. Although not shown inFIG. 1, a display element is provided between the first flexiblesubstrate 301 and the second flexible substrate 307. In other words, thepixel portion 302, the first circuit portion 304, the second circuitportion 305, and the display element are sealed with the first flexiblesubstrate 301, the sealant 312, and the second flexible substrate 307.

Furthermore, in the display device 300, an FPC terminal portion 308(FPC: flexible printed circuit) that is electrically connected to thepixel portion 302, the first circuit portion 304, and the second circuitportion 305 is provided over the first flexible substrate 301 in aregion different from a region surrounded by the sealant 312.

The FPC terminal portion 308 is connected to an FPC 316, and a varietyof signals are supplied to the pixel portion 302, the first circuitportion 304, and the second circuit portion 305 with the FPC 316. Inaddition, signal lines 310 are connected to the pixel portion 302, thefirst circuit portion 304, the second circuit portion 305, and the FPCterminal portion 308. The variety of signals supplied from the FPC 316are given to the pixel portion 302, the first circuit portion 304, andthe second circuit portion 305 through the signal lines 310.

In FIG. 1, the circuits for driving the pixel circuit portion 302 arepositioned in two regions; however, the structure of the circuit is notlimited thereto. For example, the circuit may be positioned in oneregion. Alternatively, the circuit may be divided into three or moreparts. Further alternatively, only one of the first circuit portion 304and the second circuit portion 305 may be provided over the firstflexible substrate 301, and the other circuit may be externallyprovided.

Further, the circuit for driving the pixel portion 302 may be formeddirectly on the first flexible substrate 301 like a transistor includedin pixel portion 302, or may be formed by mounting an IC chip on thefirst flexible substrate 301 by chip on glass (COG) or the like.Alternatively, the circuit may be connected to a TCP or the like.

The pixel portion 302, the first circuit portion 304, and the secondcircuit portion 305 in the display device 300 include a plurality oftransistors in which a channel formation region is formed using an oxidesemiconductor layer.

Since the transistor using an oxide semiconductor layer has highmobility, an area occupied by transistors can be made small, and theaperture ratio can be increased. With use of the transistor, the firstcircuit portion 304 and the second circuit portion 305 can be formedover the substrate provided with the pixel portion 302. In addition, thetransistor has extremely low off-state current and can hold a videosignal or the like for a longer period; thus, the frame frequency can belowered, and the power consumption of the display device can be reduced.

The oxide semiconductor layer preferably includes a c-axis alignedcrystal. In the case where the oxide semiconductor layer including thecrystal is used for a channel formation region of the transistor, acrack or the like is less likely to occur in the oxide semiconductorlayer when the display device 300 is bent, for example. As a result, thereliability can be improved.

Thus, with use of the transistor using an oxide semiconductor layer, adisplay device that is superior to a display device including anamorphous silicon layer or a polycrystalline silicon layer can beformed, for example.

As a display element included in the display device 300, a liquidcrystal element or a light-emitting element can be typically used.

Next, a display device 300 a including a liquid crystal element isdescribed. FIG. 2 is a cross-sectional view along dashed-dotted lineA1-A2 in FIG. 1 in the case where a liquid crystal element is used forthe display device 300.

The display device 300 a includes the first flexible substrate 301, afirst bonding layer 318 a, a first organic resin layer 320 a, a firstinsulating film 321 a, a first element layer, a second element layer, asecond insulating film 321 b, a second organic resin layer 320 b, asecond bonding layer 318 b, and the second flexible substrate 307, whichare stacked in this order.

In FIG. 2, the first element layer includes transistors 350 and 352,insulating films 364, 366, and 368, a planarization insulating film 370,a connection electrode 360, a conductive film 372, and the like. Thesecond element layer includes a conductive film 374, an insulating film334, a coloring layer 336 (color filter), a light-blocking layer 338(black matrix), and the like. There is a case where some of the abovecomponents is not included or a component other than the abovecomponents is included in the first element layer and the second elementlayer.

The first element layer and the second element layer are sealed with aliquid crystal layer 376 and the sealant 312 to form a liquid crystalelement 375.

Examples of the material of the first flexible substrate 301 and thesecond flexible substrate 307 include glass thin enough to haveflexibility, polyester resins such as polyethylene terephthalate (PET)and polyethylene naphthalate (PEN), a polyacrylonitrile resin, apolyimide resin, a polymethyl methacrylate resin, a polycarbonate (PC)resin, a polyethersulfone (PES) resin, a polyamide resin, a cycloolefinresin, a polystyrene resin, a polyamide imide resin, a polyvinylchloride resin, and a polyether ether ketone (PEEK) resin. Inparticular, a material whose coefficient of thermal expansion is low ispreferable, and for example, a polyamide imide resin, a polyimide resin,or PET can be suitably used. A substrate in which a glass fiber isimpregnated with an organic resin or a substrate whose thermal expansioncoefficient is reduced by mixing an organic resin with an inorganicfiller can also be used.

For the bonding layers 318 a and 318 b, for example, a resin that iscurable at room temperature such as a two-component type resin, alight-curable resin, a heat-curable resin, or the like can be used. Theexamples include an epoxy resin, an acrylic resin, a silicone resin, aphenol resin, and the like. In particular, a material with low moisturepermeability, such as an epoxy resin, is preferred.

The first organic resin layer 320 a and the second organic resin layer320 b can be formed using a material selected from an epoxy resin, anaramid resin, an acrylic resin, a polyimide resin, a polyamide resin, ora polyamide-imide resin, for example.

As the first insulating film 321 a and the second insulating film 321 b,a single layer of a silicon oxide film, a silicon oxynitride film, asilicon nitride film, or a silicon nitride oxide film, or a stackedlayer including any of the films can be used. In particular, a filmcontaining nitrogen with high blocking properties against impurities ispreferably used to prevent impurities contained in the first flexiblesubstrate 301, the first bonding layer 318 a, or the like from diffusinginto the transistor or the like.

The display device 300 a includes a lead wiring portion 311, the pixelportion 302, the first circuit portion 304, and the FPC terminal portion308. Note that the lead wiring portion 311 includes the signal line 310.

The display device 300 a has a structure in which the transistor 350 andthe transistor 352 are included in the pixel portion 302 and the firstcircuit portion 304, respectively.

In FIG. 2, the sizes of the transistor 350 and the transistor 352 arethe same; however, the sizes of the transistors are not limited thereto.The sizes of the transistor 350 and the transistor 352 can be changed(in the channel length, the channel width, and the like) as appropriate,or the number of transistors can be changed. In addition, the secondcircuit portion 305 (not shown in FIG. 2) can have a structure similarto that of the first circuit portion 304 when a portion to be connected,a connecting method, or the like is changed.

The signal line 310 included in the lead wiring portion 311 can beformed in a step of forming a source electrode layer and a drainelectrode layer of the transistor 350.

The FPC terminal portion 308 includes the connection electrode 360, ananisotropic conductive film 380, and the FPC 316. The connectionelectrode 360 can be formed in a step of forming the source electrodelayer and the drain electrode layer of the transistor 350. In addition,the connection electrode 360 is electrically connected to a terminal ofthe FPC 316 through the anisotropic conductive film 380.

A wiring containing a copper element is preferably used for the signalline connected to the transistor in the pixel portion and the transistorin the driver circuit portion. When the wiring containing a copperelement is used, the signal delay due to the wiring resistance and thelike can be suppressed.

Further, in FIG. 2, the insulating films 364, 366, and 368 and theplanarization insulating film 370 are provided over the transistor 350and the transistor 352.

The insulating films 364 and 366 can be formed using the same material,and for example, silicon oxide, silicon oxynitride, or the like can beused. The insulating film 364 is preferably formed using an oxideinsulating film with few defects, and the insulating film 366 ispreferably formed using an oxide insulating film containing oxygen inexcess of the stoichiometric composition. Note that the insulating films364 and 366 can be a single layer formed using the same material. Theinsulating film 368 has a function of blocking oxygen, hydrogen, water,an alkali metal, an alkaline earth metal, or the like. For example, anitride insulating film is preferably used.

The planarization insulating film 370 can be formed using aheat-resistant organic material, such as a polyimide resin, an acrylicresin, a polyimide amide resin, a benzocyclobutene resin, a polyamideresin, or an epoxy resin. Note that the planarization insulating film370 may be formed by stacking a plurality of insulating films formedfrom these materials. Alternatively, a structure without theplanarization insulating film 370 may be employed.

The conductive film 372 is electrically connected to one of the sourceelectrode layer and the drain electrode layer of the transistor 350. Theconductive film 372 functions as a pixel electrode formed over theplanarization insulating film 370, i.e., one electrode of the liquidcrystal element. As the conductive film 372, a conductive film havingproperties of transmitting visible light is preferably used. Forexample, a material including one of indium (In), zinc (Zn), and tin(Sn) is preferably used for the conductive film.

The liquid crystal element 375 includes the conductive film 372, theconductive film 374, and the liquid crystal layer 376. The conductivefilm 374 is provided on the second flexible substrate 307 side andfunctions as a counter electrode. In the display device 300 aillustrated in FIG. 2, an orientation state of the liquid crystal layer376 is changed by the voltage applied to the conductive film 372 and theconductive film 374, so that transmission or non-transmission of lightis changed and thus an image can be displayed.

Although not shown in FIG. 2, alignment films may be formed between theconductive film 372 and the liquid crystal layer 376 and between theconductive film 374 and the liquid crystal layer 376. An optical member(an optical substrate) such as a polarizing member, a retardationmember, or an anti-reflection member, and the like may be provided asappropriate. For example, circular polarization may be employed by usinga polarizing substrate and a retardation substrate. In addition, abacklight, a sidelight, or the like may be used as a light source.

A spacer 378 is provided between the first flexible substrate 301 andthe second flexible substrate 307. The spacer 378 is a columnar spacerobtained by selective etching of an insulating film and is provided inorder to adjust the thickness (cell gap) of the liquid crystal layer376. Note that as the spacer 378, a spherical spacer may be used.

For the liquid crystal layer 376, a liquid crystal material such asthermotropic liquid crystal, low-molecular liquid crystal,high-molecular liquid crystal, polymer dispersed liquid crystal,ferroelectric liquid crystal, or anti-ferroelectric liquid crystal canbe used. Such a liquid crystal material exhibits a cholesteric phase, asmectic phase, a cubic phase, a chiral nematic phase, an isotropicphase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric fieldmode, a liquid crystal exhibiting a blue phase for which an alignmentfilm is unnecessary may be used. A blue phase is one of liquid crystalphases, which is generated just before a cholesteric phase changes intoan isotropic phase while temperature of cholesteric liquid crystal isincreased. Since the blue phase appears only in a narrow temperaturerange, a liquid crystal composition in which several weight percent ormore of a chiral material is mixed is used for the liquid crystal layerin order to improve the temperature range. The liquid crystalcomposition containing a liquid crystal showing a blue phase and achiral material has a short response time and optical isotropy, whichmakes the alignment process unneeded and the viewing angle dependencesmall. An alignment film does not need to be provided and rubbingtreatment is thus not necessary; accordingly, electrostatic dischargedamage caused by the rubbing treatment can be prevented and defects anddamage of the liquid crystal display device in the manufacturing processcan be reduced.

In the case where the liquid crystal element is used as a displayelement, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode,a fringe field switching (FFS) mode, an axially symmetric alignedmicro-cell (ASM) mode, an optical compensated birefringence (OCB) mode,a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modeis preferable. There are some examples of a vertical alignment mode; forexample, a multi-domain vertical alignment (MVA) mode, a patternedvertical alignment (PVA) mode, an ASV mode, or the like can be employed.

As a display method in the pixel portion 302, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, a display unit may be composed of fourpixels of the R pixel, the G pixel, the B pixel, and a W (white) pixel.Alternatively, a display unit may be composed of two of color elementsamong R, G, and B as in PenTile layout. The two colors may differ amongdisplay units. Alternatively, one or more colors of yellow, cyan,magenta, and the like may be added to RGB. Further, the size of adisplay region may be different depending on respective dots of thecolor components. Embodiments of the disclosed invention are not limitedto a display device for color display; the disclosed invention can alsobe applied to a display device for monochrome display.

Next, a display device 300 b including a light-emitting element isdescribed. FIG. 3 is a cross-sectional view along dashed-dotted lineA1-A2 in FIG. 1 in the case where a light-emitting element is used forthe display device 300. Note that the same description as that of thedisplay device 300 a including the liquid crystal element is omitted.

The display device 300 b includes the first flexible substrate 301, thefirst bonding layer 318 a, the first organic resin layer 320 a, thefirst insulating film 321 a, a first element layer 410, a second elementlayer 411, the second insulating film 321 b, the second organic resinlayer 320 b, the second bonding layer 318 b, and the second flexiblesubstrate 307, which are stacked in this order.

In FIG. 3, the first element layer 410 includes the transistors 350 and352, the insulating films 364, 366, and 368, the planarizationinsulating film 370, a light-emitting element 480, an insulating film430, the signal line 310, and the connection electrode 360. The secondelement layer 411 includes the insulating film 334, the coloring layer336, and the light-blocking layer 338. The first element layer 410 andthe second element layer 411 are sealed with a sealing layer 432 and thesealant 312. Note that there is a case where part of the abovecomponents is not included or a component other than the abovecomponents is included in the first element layer 410 and the secondelement layer 411.

The light-emitting element 480 includes a conductive film 444, an ELlayer 446, and a conductive film 448. The display device 300 b enablesan image to be displayed when the EL layer 446 in the light-emittingelement 480 emits light.

The insulating film 430 is provided over the conductive film 444 overthe planarization insulating film 370. The insulating film 430 partlycovers the conductive film 444. A conductive film with high propertiesof reflecting light emitted from the EL layer is used for the conductivefilm 444, and a conductive film with high properties of transmittinglight emitted from the EL layer is used for the conductive film 448,whereby the light-emitting element 480 can have a top emissionstructure. Alternatively, a conductive film with high properties oftransmitting the light is used for the conductive film 444, and aconductive film with high properties of reflecting light is used for theconductive film 448, whereby the light-emitting element 480 can have abottom emission structure. Further alternatively, a conductive film withhigh properties of transmitting the light is used for both theconductive film 444 and the conductive film 448, whereby a dual emissionstructure can be obtained.

The coloring layer 336 is provided to overlap with the light-emittingelement 480, and the light-blocking layer 338 is provided to overlapwith the insulating film 430 and to be included in the lead wiringportion 311 and in the first circuit portion 304. The coloring layer 336and the light-blocking layer 338 are covered with a third insulatingfilm 334. A space between the light-emitting element 480 and the thirdinsulating film 334 is filled with the sealing layer 432. Although astructure with the coloring layer 336 is described as the display device300 b, the structure is not limited thereto. In the case where the ELlayer 446 is formed by a separate coloring method, the coloring layer336 is not necessarily provided.

In the display device 300 b, a dry agent may be included in the bondinglayers 318 a and 318 b. For example, a substance that absorbs moistureby chemical adsorption, such as oxide of an alkaline earth metal (e.g.,calcium oxide or barium oxide), can be used. Alternatively, a substancethat adsorbs moisture by physical adsorption, such as zeolite or silicagel, may be used. The drying agent is preferably included because entryof an impurity such as moisture into the light-emitting element 480 canbe suppressed, thereby improving the reliability of the display device.

In addition, it is preferable to mix a filler with a high refractiveindex (e.g., titanium oxide) into the sealing layer 432, in which casethe efficiency of light extraction from the light-emitting element 480can be improved.

The bonding layers 318 a and 318 b may also include a scattering memberfor scattering light. For example, the bonding layers 318 a and 318 bcan be a mixture of the sealing layer 432 and particles having arefractive index different from that of the sealing layer 432. Theparticles function as the scattering member for scattering light. Thedifference in refractive index between the sealing layer 432 and theparticles is preferably 0.1 or more, further preferably 0.3 or more. Asthe particles, titanium oxide, barium oxide, zeolite, or the like can beused. Particles of titanium oxide or barium oxide are preferable becausethey scatter light excellently. When zeolite is used, it can adsorbwater contained in the sealing layer 432 and the like, thereby improvingthe reliability of the light-emitting element.

Each of the first flexible substrate 301 and the second flexiblesubstrate 307 are preferably formed using a material with hightoughness. Thus, a light-emitting device with high impact resistancethat is less likely to be broken can be provided. For example, when thefirst flexible substrate 301 and the second flexible substrate 307 areeach an organic resin substrate, it is possible to manufacture a displaydevice that is light and less likely to be broken as compared with thecase where a glass substrate is used.

Furthermore, when a material with high thermal emissivity is used forthe first flexible substrate 301, the surface temperature of the displaydevice can be prevented from rising, leading to prevention of breakageor a decrease in reliability of the display device. For example, thefirst flexible substrate 301 may have a stacked structure of a metalsubstrate and a layer with high thermal emissivity (e.g., the layer canbe formed using a metal oxide or a ceramic material).

Next, a method for manufacturing the display device 300 b illustrated inFIG. 3 is described with reference to FIGS. 4A to 4D and FIGS. 5A to 5C.Note that to avoid complexity of the drawings, the first element layer410 and the second element layer 411 illustrated in FIG. 3 aresimplified in FIGS. 4A to 4D and FIGS. 5A to 5C.

First, over a first substrate 462, the first organic resin layer 320 a,the first insulating film 321 a, and the first element layer 410 arestacked in this order to form a stacked layer (see FIG. 4A).

Over a second substrate 463, the second organic resin layer 320 b, thesecond insulating film 321 b, and the second element layer 411 arestacked to form a stacked layer (see FIG. 4B).

The first substrate 462 and the second substrate 463 are necessary tohave at least heat resistance high enough to withstand heat treatmentperformed later. For example, a glass substrate, a ceramic substrate, aquartz substrate, a sapphire substrate, or the like can be used.

The first organic resin layer 320 a and the second organic resin layer320 b can be formed using an organic resin such as an epoxy resin, anaramid resin, an acrylic resin, a polyimide resin, a polyamide resin, ora polyamide-imide resin. In particular, a polyimide resin is preferablyused because it has high heat resistance. In the case where a polyimideresin is used, the thickness of the polyimide resin is greater than orequal to 3 nm and less than or equal to 20 μm, preferably greater thanor equal to 500 nm and less than or equal to 2 μm. The polyimide resincan be formed using a spin coating method, a dip coating method, adoctor blade method, or the like.

The first insulating film 321 a and the second insulating film 321 b canbe formed using a silicon oxide film, a silicon oxynitride film, asilicon nitride film, a silicon nitride oxide film, or the like, by asputtering method, a CVD method, or the like. With the first insulatingfilm 321 a, diffusion of impurities from the first substrate 462 or thefirst organic resin layer 320 a to the first element layer 410 can besuppressed, for example.

In formation of the first element layer 410, the temperature at whichall of the components included in the transistor 350 is formed ispreferably higher than or equal to room temperature and lower than orequal to 300° C. For example, an insulating film or a conductive filmformed using an inorganic material included in the first element layer410 is formed at the temperature higher than or equal to 150° C. andlower than or equal to 300° C., preferably higher than or equal to 200°C. and lower than or equal to 270° C. An insulating film or the likeformed using an organic resin material included in the first elementlayer 410 is preferably formed at the temperature higher than or equalto room temperature and lower than or equal to 100° C. In formation ofthe transistor 350, for example, a heating step can be skipped.

The insulating film 430, the conductive film 444, the EL layer 446, andthe conductive film 448 included in the first element layer 410 can beformed by the following method.

For the insulating film 430, an organic resin or an inorganic insulatingmaterial can be used, for example. As the organic resin, for example, apolyimide resin, a polyamide resin, an acrylic resin, a siloxane resin,an epoxy resin, a phenol resin, or the like can be used. As theinorganic insulating material, silicon oxide, silicon oxynitride, or thelike can be used, for example. There is no particular limitation on themethod for forming the insulating film 430. A photolithography method, asputtering method, an evaporation method, a droplet discharging method(e.g., an inkjet method), a printing method (e.g., a screen printingmethod or an offset printing method), or the like can be used. When theinsulating film 430 is formed using a photosensitive resin by aphotolithography method, a formation step can be simplified.

As the conductive film 444, a metal film with high reflectance ofvisible light is preferably used. As the metal film, for example,aluminum, silver, an alloy thereof, or the like can be used. Theconductive film 444 can be formed by a sputtering method, for example.

For the EL layer 446, a light-emitting material that can emit light byrecombining holes and electron injected from the conductive film 444 andthe conductive film 448 may be used. In addition to the light-emittingmaterial, a functional layer such as a hole injection layer, a holetransport layer, an electron transport layer, or an electron injectionlayer may be formed as needed. Furthermore, the EL layer 446 can beformed, for example, by an evaporation method, a coating method, or thelike.

As the conductive film 448, a conductive film having properties oftransmitting visible light is preferably used. For example, a materialincluding one of indium (In), zinc (Zn), and tin (Sn) is preferably usedfor the conductive film. For the conductive film 448, alight-transmitting conductive material such as indium oxide includingtungsten oxide, indium zinc oxide including tungsten oxide, indium oxideincluding titanium oxide, indium tin oxide including titanium oxide,indium tin oxide (ITO), indium zinc oxide, or indium tin oxide to whichsilicon oxide is added can be used. In the display device according toone embodiment of the present invention, indium tin oxide to whichsilicon is added is preferably used for the conductive film 448. Whenthe indium tin oxide to which silicon is added is used, the resistanceto bending of the conductive film 448 is improved, crack or the like isless likely to occur. The conductive film 448 can be formed by asputtering method, for example.

The coloring layer 336 in the second element layer 411 is a coloringlayer that transmits light in a specific wavelength range. For example,a red (R) color filter for transmitting light in a red wavelength range,a green (G) color filter for transmitting light in a green wavelengthrange, a blue (B) color filter for transmitting light in a bluewavelength range, or the like can be used. Alternatively, a yellow (Y)color filter or a white (W) color filter may be used. Each color filteris formed in a desired position with any of various materials by aprinting method, an inkjet method, an etching method using aphotolithography technique, or the like.

The light-blocking layer 338 in the second element layer 411 has afunction of blocking light in a particular wavelength range, and can beformed using a metal film, an organic insulating film including a blackpigment, or the like.

As the third insulating film 434 in the second element layer 411, anorganic insulating film including an acrylic resin or the like can beused, for example. Note that the third insulating film 434 is notnecessarily formed, and a structure without the third insulating film434 may be used.

Next, the first element layer 410 and the second element layer 411 arebonded with the sealing layer 432 provided therebetween (see FIG. 4C).Note that the sealant 312 is not shown.

For the sealing layer 432, a solid sealing material with flexibility canbe used. For example, a glass material such as a glass frit, or a resinmaterial such as a two-component-mixture-type resin which is curable atroom temperature, a light curable resin, a thermosetting resin, and thelike can be used.

Next, the first substrate 462 is separated from the structureillustrated in FIG. 4C (see FIG. 4D). Note that a step of separating thesecond substrate 463 may be performed prior to the separation of thefirst substrate 462.

For the separation step, a variety of methods can be employed asappropriate. For example, the first substrate 462 can be separated inthe following manner: by irradiating the first organic resin layer 320 awith ultraviolet light 468 through the first substrate 462, the firstorganic resin layer 320 a is weakened; or the adhesion between the firstorganic resin layer 320 a and the first substrate 462 is lowered.Alternatively, the irradiation with the ultraviolet light 468 may beperformed under such a condition that the irradiation energy density isadjusted, so that a region with a high adhesion between the firstsubstrate 462 and the first organic resin layer 320 a and a region witha low adhesion between the first substrate 462 and the first organicresin layer 320 a are formed, and then, the first substrate 462 may beseparated. As a light source of ultraviolet light, for example, anexcimer laser that emits ultraviolet light with a wavelength of 308 nmcan be used. Alternatively, a high-pressure mercury lamp, an UV-LED, orthe like may be used.

The excimer laser is a pulsed laser with high output, which can shape abeam into a linear form with an optical system. The substrate is movedwithin a range that is irradiated with a linear laser bean, so that thewhole or necessary portion of the substrate can be irradiated with laserlight. Note that when the length of linear beam is longer than or equalto one side of the substrate, the substrate is moved only in onedirection, so that the whole substrate can be irradiated with laserlight.

For an excimer laser device, besides a device on which one laseroscillator is mounted, a device on which two or more laser oscillatorsare mounted may be used. In the device including a plurality of laseroscillators, laser beams that are outputted in synchronization from thelaser oscillators are synthesized (superimposed) with an optical system,so that laser light with high energy density can be obtained. Thus, inthe application according to this embodiment, a substrate whose size islarger than or equal to the 8th generation glass substrate (2160 mm×2460mm) can be processed. In the device including a plurality of laseroscillators, the output variation of laser light emitted from the laseroscillators is compensated, so that a variation in intensity per pulseis reduced, and a high yield process can be performed. Instead of adevice including a plurality of laser oscillators, a plurality ofexcimer laser devices can be used.

FIG. 6 illustrates an example of a processing device using an excimerlaser. Laser light 610 a and 610 b emitted from an excimer laser device600 having two laser oscillators is synthesized by an optical system630. Laser light 610 c that is extended horizontally by the opticalsystem 630 is incident on a lens 670 via a mirror 650 and condensed intoa linear beam 610 d. At this time, a processing region 710 in aprocessing object 700 is irradiated with the linear beam 610 d through asubstrate 720.

In this embodiment, the processing object 700 corresponds to a structureillustrated in FIG. 4C or FIG. 5A, the processing region 710 correspondsto the first organic resin layer 320 a or the second organic resin layer320 b, and the substrate 720 corresponds to the first substrate 462 orthe second substrate 463.

The processing object 700 is moved in the direction of an arrow in thedrawing, whereby the whole of processing region 710 can be irradiatedwith the linear beam 610 d. As the excimer laser, a laser whosewavelength is 308 nm or longer is preferably used. With a laser with awavelength of 308 nm or longer, the sufficient amount of laser lightthat is necessary for processing can be transmitted through thesubstrate 720 even when a glass substrate is used for the substrate 720.

In this embodiment, a method for performing separation at an interfacebetween the first substrate 462 and the first organic resin layer 320 ais described; however, a separation method is not limited thereto. Forexample, separation may be performed so that part of the first organicresin layer 320 a is left on the first substrate 462 after separation.Alternatively, separation may be performed at an interface between thefirst organic resin layer 320 a and the first element layer 410.

Further alternatively, the first organic resin layer 320 a may beseparated from the first substrate 462 by injecting a liquid into theinterface between the first substrate 462 and the organic resin layer320 a. Further alternatively, the first element layer 410 may beseparated from the first organic resin layer 320 a by injecting a liquidinto the interface between the first organic resin layer 320 a and thefirst element layer 410. As the liquid, water, a polar solvent, or thelike can be used, for example. With the liquid, static electricity thatis caused by separation can be suppressed, and the electrostaticbreakdown of the transistor in the first element layer 410 or the likecan be suppressed.

Next, the first organic resin layer 320 a and the first flexiblesubstrate 301 are bonded with the first bonding layer 318 a (see FIG.5A).

Next, the second substrate 463 is separated by a method similar to theabove method, and the second organic resin layer 320 b and the secondflexible substrate 307 are bonded with the second bonding layer 318 b(see FIG. 5B).

Then, the FPC 316 is bonded to the connection electrode 360 with theanisotropic conductive film 380. An IC chip or the like may be mountedif necessary.

According to the above steps, the display device 300 b illustrated inFIG. 3 can be manufactured.

In the case where separation is performed at the interface between thefirst organic resin layer 320 a and the first element layer 410, thestructure illustrated in FIG. 5C is obtained. In this case, in thedisplay devices 300 a and 300 b illustrated in FIG. 2 and FIG. 3, theorganic resin layer 320 a does not exist.

In one embodiment of the present invention, an element layer including atransistor using an oxide semiconductor layer and the like is formedover an organic resin layer, and the organic resin layer is weakened orthe adhesion between the organic resin layer and the substrate isreduced, whereby the element layer is separated. In the case where thetransistor includes polycrystalline silicon, a laser irradiation step isperformed to crystallize amorphous silicon. In the laser irradiationstep, a region at which the temperature becomes so high that silicon ismelted is instantaneously generated. Thus, in the case where an organicresin layer is used like one embodiment of the present invention, heatis conducted to the organic resin layer, and a crack or peeling mayoccur, due to degassing or thermal expansion, in an inorganic filmformed between the transistor and the substrate. In the case of theirradiation with a laser whose energy density is made low to suppressthe occurrence of crack or peeling, polycrystalline silicon havingsufficient crystallinity cannot be obtained.

In contrast, in a process of manufacturing a transistor using an oxidesemiconductor layer, a high temperature step is not needed, and theprocess up to completion of the transistor or the like can be performedstably without weakening of the organic resin layer; thus, a transistorwith high yield and reliability can be formed.

The structure described in this embodiment can be used in appropriatecombination with structure described in any of the other embodiments.

Embodiment 2

In this embodiment, a separation apparatus of one embodiment of thepresent invention will be described with reference to FIGS. 7A to 7D,FIGS. 8A to 8C, FIGS. 9A to 9C, FIGS. 10A to 10E, FIGS. 11A to 11C,FIGS. 12A to 12C, FIGS. 13A to 13C, and FIGS. 14A and 14B. Oneembodiment of the present invention is a separation apparatus includinga structure body that can hold a first member of a processed member, anda stage that can hold a second member of the processed member. With theseparation apparatus, the processed member between the structure bodyand the stage is separated into the first member and the second memberwhile the first member is rolled up. For example, the first membercorresponds to the stacked body other than the second substrate 463illustrated in FIG. 5A, and the second member corresponds to the secondsubstrate 463.

With the separation apparatus of one embodiment of the presentinvention, the processed member can be separated into the first memberand the second member efficiently. The separation apparatus of oneembodiment of the present invention has no complicated structure and canbe applied to separation of processed members that have a variety ofsizes.

A structure and operation of a separation apparatus and a separationmethod with use of the separation apparatus are described below.

Structure Example 1

With reference to FIGS. 7A to 7D, FIGS. 8A to 8C, and FIGS. 9A to 9C, anexample in which a first member 103 a is peeled from a processed member103 to separate the first member 103 a and a second member 103 b isshown.

FIG. 7A is a perspective view of a separation apparatus just before thepeeling is performed, FIG. 7B is a front view thereof, and FIG. 7D is aside view thereof.

The separation apparatus illustrated in FIGS. 7A to 7D includes astructure body 101 and a stage 105. The structure body 101 has a convexsurface. The stage 105 has a supporting surface facing the convexsurface.

In FIGS. 7A to 7D, the processed member 103 is arranged between theconvex surface and the supporting surface of the separation apparatus.

FIG. 7C is a top view showing a case where a position of the processedmember 103 with respect to the structure body 101 is different from thatin FIG. 7A, FIG. 7B, and FIG. 7D. FIG. 7A shows the case where thepeeling starts at a side portion of the processed member 103.Alternatively, the peeling may start at the corner portion of theprocessed member 103 as shown in the top view of FIG. 7C. In the casewhere the peeling starts at the side portion of the processed member103, the short side is peeled, and the peeling preferably proceeds inthe long side direction. Thus, the condition of rotation speed of thestructure body is easily adjusted, and the separation yield can beincreased.

The processed member 103 has a sheet-like shape, which includes thesheet-like first member 103 a and the sheet-like second member 103 b.Each of the first member 103 a and the second member 103 b may be asingle layer or a stacked layer. In the processed member 103, a peelingtrigger is preferably formed. With the trigger, the peeling is easilyperformed at the interface between the first member 103 a and the secondmember 103 b.

In the case where the separation apparatus includes a transfer unit, theprocessed member 103 may be provided over the stage 105 with use of thetransfer unit.

As shown in an enlarged view corresponding to a portion surrounded by adashed-two dotted line in FIG. 7D, the convex surface of the structurebody 101 is overlapped with a peeling trigger 102 with a point-likeshape or linear (including a solid line, a dashed line, or a frameshape) formed in the processed member 103. Then, force for peeling thefirst member 103 a is applied to the processed member 103 by rotation ofthe structure body 101, and peeling of the first member 103 a starts inthe vicinity of the peeling trigger 102. After that, the processedmember 103 is separated into the first member 103 a and the secondmember 103 b.

The structure body 101 preferably has a convex shape, for example, acylindrical shape (including a columnar shape, a right cylinder shape,an elliptical cylinder shape, a parabolic cylinder shape, and the like),a spherical shape, a structure whose part is a cylinder shape, astructure whose part is a spherical shape, or the like. For example, aroller such as a drum roller can be used.

As a material of the structure body, a metal, an alloy, an organicresin, or the like can be given. The structure body may have a space orhole inside.

FIGS. 10C and 10D illustrate a structure body 151 and a structure body152 each of which partly has a convex surface. Each of the structurebody 151 and the structure body 152 partly has a cylindrical shape.

The curvature radius of the convex surface of the structure body issmaller than that of the supporting surface of the stage 105. Thecurvature radius of the convex surface can be, for example, greater thanor equal to 0.5 mm and less than or equal to 1000 mm. For example, whenthe processed member is separated, the curvature radius of the convexsurface may be greater than or equal to 0.5 mm and less than or equal to500 mm, specifically 150 mm, 225 mm, 300 mm, or the like. Examples ofstructure bodies having such a convex surface include a roller whosediameter is 300 mm, 450 mm, or 600 mm. Note that the preferable range ofthe curvature radius of the convex surface varies depending on thethickness or size of the processed member. Thus, the curvature radius ofthe structure body is not limited to the above, and it is preferablethat the curvature radius of the structure body be smaller than that ofthe supporting surface of the stage 105.

In the case where the processed member 103 has a stacked structure withlow adhesion, separation occurs at the interface with low adhesion, andthe yield of the separation may be reduced. For example, in the casewhere the processed member 103 includes an organic EL element,separation occurs at an interface between two layers included in an ELlayer or an interface between the EL layer and an electrode, andaccordingly, separation at an interface between the first member 103 aand the second member 103 b is difficult in some cases. Thus, thecurvature radius of the convex surface is determined so as to cause theseparation at the interface between the first member 103 a and thesecond member 103 b. Alternatively, the separation portion may beadjusted with the rotation speed of the structure body 101.

When the curvature radius of the convex surface is too small, an elementincluded in the first member 103 a rolled up on the convex surface maybe broken. Thus, the curvature radius of the convex surface ispreferably greater than or equal to 0.5 mm.

When the curvature radius of the convex surface is large, a substratewith low flexibility and high stiffness, such as a glass substrate, asapphire substrate, a quartz substrate, or a silicon substrate, can berolled up on the convex surface. Thus, the curvature radius of theconvex surface is preferably greater than or equal to 300 mm.

Furthermore, when the curvature radius of the convex surface is large, aseparation apparatus has a large size, and an installation site or thelike may be restricted. Thus, the curvature radius of the convex surfaceis preferably less than or equal to 1000 mm, further preferably lessthan or equal to 500 mm, for example.

At least part of the convex surface may have viscosity. For example, anadhesive tape or the like may be put on part or the whole of the convexsurface. As illustrated in FIG. 10E, at least part of the convex surfacemay be provided with a portion 104 having viscosity to stick to thefirst member 103 a. Alternatively, the structure body 101 itself mayhave an adsorbing mechanism, so that the convex surface can be attachedto the first member 103 a.

The structure body 101 or the stage 105 may be movable in at least anyof the following directions: forward and backward; right and left; andup and down. It is preferable that the distance between the convexsurface of the structure body 101 and the supporting surface of thestage 105 be adjustable for the separation of the processed members withvaried thicknesses. In Structure Example 1, the structure body 101 canbe moved in the longitudinal direction of the stage 105.

As a holding unit for holding a member or the like (e.g., the processedmember 103 or the second member 103 b) placed over the stage 105, achuck such as a suction chuck, an electrostatic chuck, or a mechanicalchuck can be given. For example, a porous chuck may be used.Alternatively, a member may be fixed to a suction table, a heater table,a spinner table, or the like.

FIG. 8A is a perspective view of a separation apparatus in a state wherethe peeling is being conducted, FIG. 8B is a front view thereof, andFIG. 8C is a side view thereof. FIG. 9A is a perspective view of theseparation apparatus after the peeling step, FIG. 9B is a front viewthereof, and FIG. 9C is a side view thereof.

A rotation axis 109 is provided at the center of the structure body 101.Although the direction in which the structure body 101 is rotated isshown in FIGS. 8A, 8C, and the like, the structure body 101 may berotated in the reverse direction as well as the direction shown in FIGS.8A, 8C, and the like. In addition, the rotation axis 109 is moved alonga groove of a guide 107, whereby the structure body 101 can be moved inthe longitudinal direction of the stage 105 (sideways in FIG. 8C andFIG. 9C)

By rotation of the structure body 101, the peeling of the first member103 a overlapping with the convex surface of the structure body 101 fromthe processed member 103 starts in the vicinity of the peeling trigger,and the first member 103 a is separated from the second member 103 bwhile being rolled up on the convex surface. The first member 103 a isheld on the convex surface of the structure body 101, and the secondmember 103 b is held over the stage 105.

In the separation apparatus of one embodiment of the present invention,a position of the rotation center of the structure body 101 with respectto the stage 105 may be changed by moving at least one of the stage 105and the structure body 101. In Structure Example 1, an example in whichthe rotation center of the structure body 101 is moved is shown.Specifically, shown is an example in which the structure body 101 can bemoved (rotated) from one of end portions of the processed member 103toward the opposite end portion while the structure body 101 is rollingup the first member 103 a in a state where the stage 105 is stationary(or fixed).

The linear speed of the convex surface of the structure body 101 ishigher than or equal to the moving speed of the rotation center of thestructure body 101 with respect to the stage 105.

The first member 103 a and the second member 103 b may be separated fromeach other while the tension is applied to the first member 103 a or thesecond member 103 b.

As indicated by an arrow 108 in FIG. 8C, a liquid supplying unit thatcan supply a liquid to a surface where the first member 103 a and thesecond member 103 b are separated from each other may be provided.

Furthermore, an adverse effect on an element or the like included in thefirst member 103 a due to static electricity caused at peeling (e.g., aphenomenon in which a semiconductor element is damaged by staticelectricity) can be suppressed. Note that a liquid may be sprayed in anatomized form or in a vaporized form. As the liquid, pure water or anorganic solvent may be used. A neutral, alkaline, or acid aqueoussolution or an aqueous solution in which salt is dissolved may be used.

In the case where the separation apparatus includes a transfer unit, thesecond member 103 b over the stage 105 or the first member 103 a rolledup on the structure body 101 may be transferred with the transfer unitafter the separation.

As illustrated in FIGS. 10A and 10B, by further rotating the structurebody 101, a sheet-like member 111 provided over the stage 105 and thefirst member 103 a may be bonded.

The member 111 may have a single-layer structure or a stacked structure.At least part of a surface of the member 111 that is in contact with thefirst member 103 a preferably has adhesiveness to the first member 103a. For example, a bonding layer may be formed.

While the structure body 101 rotates one turn, the first member 103 amay be wholly rolled up on the convex surface. This is a preferablestep, which can prevent the first member 103 a from being in contactwith the stage 105 and being pressed by the structure body 101.

Furthermore, it is preferable that the first member 103 a rolled up bebonded to the member 111 without being in contact with the stage 105.

For example, a rotation step may be performed in the following manner.First, the structure body 101 is one-fourth rotated so that the whole ofthe first member 103 a is rolled up on the convex surface. Next, thestructure body 101 is three-fourth rotated so that the structure body101 is moved to the vicinity of the end portion of the member 111. Then,the structure body 101 is one-fourth rotated so that the first member103 a is bonded on the member 111.

Alternatively, after the separation, the distance between the structurebody 101 and the stage 105 may be adjusted so that the first member 103a rolled up on the structure body 101 is not in contact with the stage105.

Structure Example 2

In Structure Example 2, an example in which a position of the rotationcenter of a structure body with respect to a stage is changed by movingthe stage is shown. Specifically, shown is an example in which theposition of the rotation center of the structure body is not moved butthe stage is moved from one end portion of a processed member toward theopposite end portion.

With reference to FIGS. 11A to 11C, FIGS. 12A to 12C, and FIGS. 13A to13C, an example in which a first member 153 a and a second member 153 bare separated from each other by peeling the first member 153 a from aprocessed member 153 is shown.

FIG. 11A is a perspective view of a separation apparatus just before thepeeling is performed, FIG. 11B is a front view thereof, and FIG. 11C isa side view thereof.

The separation apparatus illustrated in FIGS. 11A to 11C includes thestructure body 151, a stage 155, a support 157, and a transfer roller158. The structure body 151 has a convex surface. The stage 155 has asupporting surface facing the convex surface. The support 157 supportsthe structure body 151.

In FIGS. 11A to 11C, the processed member 153 is provided between theconvex surface and the supporting surface of the separation apparatus.

Although FIG. 11A shows an example in which the peeling starts at theside portion of the processed member 153, the peeling may start at thecorner portion of the processed member 153, which is similar to the casein Structure Example 1.

The structure body 151, the processed member 153, and the stage 155 canhave structures similar to those of the structure body 101, theprocessed member 103, and the stage 105 in Structure Example 1,respectively; thus, the description thereof is omitted. In the processedmember 153, a peeling trigger 162 is provided.

The support 157 supports a rotation axis 159 of the structure body 151.The support 157 has a function of adjusting the vertical position of thestructure body 151. Thus, the distance between the convex surface of thestructure body 151 and the supporting surface of the stage 155 can beadjustable.

The transfer roller 158 enables the stage 155 to be moved. There is noparticular limitation on a unit for moving the stage 155, and a conveyorbelt or a transfer robot may be used.

In the case where the separation apparatus includes a transfer unit, theprocessed member 153 may be provided over the stage 155 with thetransfer unit.

FIG. 12A is a perspective view of the separation apparatus in a statewhere the peeling is being conducted, FIG. 12B is a front view thereof,and FIG. 12C is a side view thereof. FIG. 13A is a perspective view ofthe separation apparatus after the peeling, FIG. 13B is a front viewthereof, and FIG. 13C is a side view thereof.

The rotation axis 159 is provided at the center of the structure body151. Although the direction in which the structure body 151 or thetransfer roller 158 is rotated is shown in FIGS. 12A, 12C, and the like,the structure body 151 or the transfer roller 158 may be rotated in thereverse direction as well as the direction shown in FIGS. 12A, 12C, andthe like. By rotating the transfer roller 158, the positions of thestage 155 and the processed member 153 over the stage 155 with respectto the rotation center of the structure body 151 can be moved(specifically, moved sideways in FIG. 12C or FIG. 13C).

The first member 153 a held on the structure body 151 is peeled from theprocessed member 153, rolled up on the convex surface, and separatedfrom the second member 153 b. Over the stage 155, the second member 153b is held.

The convex surface of the structure body 151 is overlapped with thepeeling trigger 162 in the processed member 153. Then, the structurebody 151 is rotated, whereby a force for peeling the first member 153 ais applied to the processed member 153, and the first member 153 a inthe vicinity of the peeling trigger 162 is peeled. The first member 153a peeled from the processed member 103 is rolled up on the convexsurface and separated from the second member 103 b. The first member 153a is held on the convex surface of the structure body 151, and thesecond member 153 b is held over the stage 155.

In the case where the separation apparatus includes a transfer unit, thesecond member 153 b over the stage 155 or the first member 153 a rolledup on the structure body 151 may be transferred with the transfer unitafter the separation.

As illustrated in FIGS. 14A and 14B, the structure body 151 and thetransfer roller 158 are rotated, whereby a sheet-like member 161provided over a stage 156 and the first member 153 a may be bonded. Notethat the member 161 may be provided over the stage 155 where theprocessed member 153 has been provided.

Structure Example 3

Another structure of a separation apparatus of one embodiment of thepresent invention is described with reference to FIGS. 15A1, 15A2, 15B1,15B2, 15C1, and 15C2. FIGS. 15A1, 15A2, 15B1, 15B2, 15C1, and 15C2illustrate a structure and operation of the separation apparatus of oneembodiment of the present invention.

FIG. 15A1, FIG. 15B1, and FIG. 15C1 are schematic views eachillustrating a side surface of the separation apparatus of oneembodiment of the present invention.

FIG. 15A2, FIG. 15B2, and FIG. 15C2 are schematic views eachillustrating a top surface of the separation apparatus.

FIG. 15A1 and FIG. 15A2 illustrate the separation apparatus of oneembodiment of the present invention in a state where a step of peelingthe first member 103 a from the processed member 103 starts.

FIG. 15B1 and FIG. 15B2 illustrate the separation apparatus of oneembodiment of the present invention in a state where the first member103 a is being peeled from the processed member 103.

FIG. 15C1 and FIG. 15C2 illustrate the separation apparatus of oneembodiment of the present invention in a state where the peeling of thefirst member 103 a from the processed member 103 is finished.

The separation apparatus described in Structure Example 3 of oneembodiment includes the cylindrical structure body 101 and a rotator 101a that is in contact with an inner wall of the cylindrical structurebody 101 and can rotate in synchronization with the rotation of thestructure body 101, which is a difference from the separation apparatusdescribed with reference to FIGS. 7A to 7D, FIGS. 8A to 8C, FIGS. 9A to9C, FIGS. 10A to 10E, FIGS. 11A to 11C, FIGS. 12A to 12C, FIGS. 13A to13C, and FIGS. 14A and 14B. A different component will be described indetail below, and the above description is referred to for the othersimilar components.

The structure body 101 has a cylindrical shape. Note that the structurebody 101 may be provided with a member 101 b on its outer surface (seeFIG. 15A1 and FIG. 15A2).

The member 101 b can modify physical properties of the surface of thestructure body 101. For example, the member 101 b allows the surface ofthe structure body 101 to have viscosity. Alternatively, the member 101b allows the surface of the structure body 101 to have elasticity thatcan disperse stress concentrated on the convex and concave portions.

For example, rubber, silicone rubber, a resin, a natural material, orthe like can be used for the member 101 b.

In the case where the member 101 b provided on the structure body 101has a joint portion, the processed member is provided between the stage105 and the structure body 101 so as to prevent the processed member 103from being in contact with the joint portion.

The rotator 101 a is in contact with the inner periphery of thecylindrical structure body 101, and the processed member 103 issandwiched between the outer periphery of the structure body 101 and thestage 105.

The rotator 101 a is provided rotatably around the center axis. Forexample, the rotator 101 a may be provided with a cylindrical roller ora gear on its outer surface.

In the case where the rotator 101 a provided with a gear on its outerperiphery is used, a gear that meshes the gear provided with the rotator101 a is provided on an inner surface of the structure body 101. In thisstructure, for example, the rotator 101 a is driven to rotate with useof a driver unit, and the rotation can influence the structure body 101.

As a first step, the processed member 103 in which the peeling trigger102 is provided is inserted between the stage 105 and the structure body101 (see FIG. 15A1 and FIG. 15A2). In the case where the processedmember 103 has a short side and a long side, the peeling trigger 102 ispreferably provided at a corner portion, and the processed member 103may be inserted from the corner portion in a state of being inclined ata B degree angle with respect to the direction perpendicular to thecenter axis of the rotator 101 a seen from above. As a result, thepeeling of the first member 103 a from the second member 103 b cangradually proceed from the peeling trigger 102.

As a second step, the further peeling of the first member 103 a from thesecond member 103 b makes progress (see FIG. 15B1 and FIG. 15B2).

With use of the liquid supply unit indicated by the arrow 108, a liquidis supplied to a surface where the first member 103 a and the secondmember 103 b are separated (separation surface) (see FIG. 15B1). Forexample, the liquid is injected into the separation surface.Alternatively, the liquid may be sprayed.

For example, as the liquid injected or sprayed, water, a polar solvent,or the like can be used. By injecting the liquid, an influence of staticelectricity and the like caused by the peeling can be reduced.Alternatively, the peeling may proceed while the peeling layer isdissolved with a liquid.

As a third step, the first member 103 a and the second member 103 b areseparated (see FIG. 15C1 and FIG. 15C2).

Structure Example 4

Another structure of a separation apparatus of one embodiment of thepresent invention is described with reference to FIGS. 16A1, 16A2, 16B1,16B2, 16C1, and 16C2. FIGS. 16A1, 16A2, 16B1, 16B2, 16C1, and 16C2illustrate a structure and operation of the separation apparatus of oneembodiment of the present invention.

FIG. 16A1, FIG. 16B1, and FIG. 16C1 are schematic views eachillustrating a side surface of the separation apparatus of oneembodiment of the present invention. FIG. 16A2, FIG. 16B2, and FIG. 16C2are schematic views each illustrating a top surface thereof.

FIG. 16A1 and FIG. 16A2 illustrate the separation apparatus of oneembodiment of the present invention in a state where the peeling of thefirst member 153 a from the processed member 153 starts.

FIG. 16B1 and FIG. 16B2 illustrate the separation apparatus of oneembodiment of the present invention in a state where the first member153 a is being peeled from the processed member 153.

FIG. 16C1 and FIG. 16C2 illustrate the separation apparatus of oneembodiment of the present invention in a state where the peeling of thefirst member 153 a from the processed member 153 is finished.

The separation apparatus described in Structure Example 4 in thisembodiment includes the cylindrical structure body 101, instead of thecylindrical structure body 151, and the rotator 101 a that is in contactwith an inner wall of the cylindrical structure body 101 and can rotatein synchronization with the rotation of the structure body 101, which isa difference from the separation apparatus described with reference toFIGS. 11A to 11C, FIGS. 12A to 12C, and FIGS. 13A to 13C.

In addition, in the separation apparatus described in Structure Example4, the structure body 151 is fixed instead of the structure body 101,and the stage 155 is moved, which is a difference from the case of theseparation apparatus described with reference to FIGS. 15A1, 15A2, 15B1,15B2, 15C1, and 15C2.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 3

In this embodiment, a structure of a display device and a displayelement of one embodiment of the present invention will be described.

The display device illustrated in FIG. 17A includes a region includingpixels of display elements (hereinafter, the region is referred to aspixel portion 502), a circuit portion being provided outside the pixelportion 502 and including a circuit for driving the pixels (hereinafter,the portion is referred to as driver circuit portion 504), circuits eachhaving a function of protecting an element (hereinafter, the circuitsare referred to as protection circuits 506), and a terminal portion 507.Note that the protection circuits 506 are not necessarily provided.

Part or all of the driver circuit portion 504 is preferably formed overthe same substrate as the pixel portion 502. Thus, the number ofcomponents and the number of terminals can be reduced. In the case wherea part or the whole of the driver circuit portion 504 is not providedover the same substrate as the pixel portion 502, an IC chip may bemounted with COG or tape automated bonding (TAB).

The pixel portion 502 includes circuits for driving the plurality ofdisplay elements in X (X is a natural number of 2 or more) rows and Ycolumns (Y is a natural number of 2 or more) (hereinafter, such circuitsare referred to as pixel circuit 501). The driver circuit portion 504includes driver circuits such as a circuit for supplying a signal (scansignal) to select a pixel (hereinafter, the circuit is referred to as agate driver 504 a) and a circuit for supplying a signal (data signal) todrive a display element in a pixel (hereinafter, the circuit is referredto as a source driver 504 b).

The gate driver 504 a includes a shifter register or the like. The gatedriver 504 a receives a signal for driving the shift register andoutputs a signal through a terminal portion 507. For example, the gatedriver 504 a receives a start pulse signal, a clock signal, or the likeand outputs a pulse signal. The gate driver 504 a has a function ofcontrolling potentials of wirings supplied with scan signals(hereinafter, such wirings are referred to as scan lines GL_1 to GL_X).Note that the plurality of gate drivers 504 a may be provided toseparately control the scan lines GL_1 to GL_X Alternatively, the gatedriver 504 a has, but is not limited to, a function of supplying aninitialization signal. The gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. Thesource driver 504 b receives a signal (video signal) from which a datasignal is derived, as well as a signal for driving the shift register,through the terminal portion 507. The source driver 504 b has a functionof generating data signals written in the pixel circuits 501 based onthe video signals. In addition, the source driver 504 b has a functionof controlling output of a data signal in response to a pulse signalproduced by input of a start pulse, a clock signal, or the like.Further, the source driver 504 b has a function of controlling thepotentials of wirings supplied with data signals (hereinafter, suchwirings are referred to as data lines DL_1 to DL_Y). Alternatively, thesource driver 504 b has, but is not limited to, a function of supplyingan initialization signal. The source driver 504 b can supply anothersignal.

The source driver 504 b includes a plurality of analog switches or thelike, for example. The source driver 504 b can output, as the datasignals, signals obtained by time-dividing the video signal bysequentially turning on the plurality of analog switches. The sourcedriver 504 b may be formed using a shift register or the like.

A pulse signal and a data signal are input to each of the plurality ofpixel circuit portions 501 through one of the plurality of scan lines GLsupplied with scan signals and one of the plurality of data lines DLsupplied with data signals, respectively. Writing and holding of thedata signal in each of the plurality of pixel circuits 501 are performedby the gate driver 504 a. For example, to the pixel circuit 501 in m-throw and n-th column (m is a natural number of less than or equal to X,and n is a natural number of less than or equal to Y), a pulse signal isinput from the gate driver 504 a through the scan line GL_m, and a datasignal is input from the source driver 504 b through the data line DL_ndepending on the potential of the scan line GL_m.

The protection circuit 506 shown in FIG. 17A is connected to, forexample, the scan line GL between the gate driver 504 a and the pixelcircuit 501. Alternatively, the protection circuit 506 is connected tothe data line DL making the connection between the source driver 504 band the pixel circuit 501. Alternatively, the protection circuit 506 canbe connected to a wiring between the gate driver 504 a and the terminalportion 507. Alternatively, the protection circuit 506 can be connectedto a wiring between the source driver 504 b and the terminal portion507. Note that the terminal portion 507 means a portion having terminalsfor inputting power, control signals, and video signals to the displaydevice from external circuits.

The protection circuit 506 is a circuit which electrically connects awiring connected to the protection circuit to another wiring when apotential out of a certain range is applied to the wiring connected tothe protection circuit.

As illustrated in FIG. 17A, the protection circuit portions 506 areprovided for the pixel portion 502 and the driver circuit portion 504,so that the resistance of the display device to overcurrent generated byelectrostatic discharge (ESD) or the like can be improved. Note that theconfiguration of the protection circuits 506 is not limited to that, andfor example, a configuration in which the protection circuits 506 areconnected to the gate driver 504 a or a configuration in which theprotection circuits 506 are connected to the source driver 504 b may beemployed. Alternatively, the protection circuit 506 may be configured tobe connected to the terminal portion 507.

In the non-limiting example illustrated in FIG. 17A, the driver circuitportion 504 includes the gate driver 504 a and the source driver 504 b.For example, only the gate driver 504 a is formed, and a source drivercircuit (e.g., IC chip or the like) which is separately prepared may bemounted.

For example, in this specification and the like, a display element, adisplay device which is a device including a display element, alight-emitting element, and a light-emitting device which is a deviceincluding a light-emitting element can employ a variety of modes or caninclude a variety of elements. Examples of a display element, a displaydevice, a light-emitting element, or a light-emitting device include adisplay medium whose contrast, luminance, reflectance, transmittance, orthe like is changed by electromagnetic action, such as anelectroluminescence (EL) element (e.g., an EL element including organicand inorganic materials, an organic EL element, or an inorganic ELelement), an LED (e.g., a white LED, a red LED, a green LED, or a blueLED), a transistor (a transistor that emits light depending on current),an electron emitter, a liquid crystal element, electronic ink, anelectrophoretic element, a grating light valve (GLV), a plasma displaypanel (PDP), a display element using micro electro mechanical system(MEMS), a digital micromirror device (DMD), a digital micro shutter(DMS), MIRASOL (registered trademark), an interferometric modulatordisplay (IMOD) element, a MEMS shutter display element, anoptical-interference-type MEMS display element, an electrowettingelement, a piezoelectric ceramic display, or a carbon nanotube. Notethat examples of display devices having EL elements include an ELdisplay. Display devices having electron emitters include a fieldemission display (FED), an SED-type flat panel display (SED:surface-conduction electron-emitter display), and the like. Examples ofdisplay devices including liquid crystal elements include a liquidcrystal display (e.g., a transmissive liquid crystal display, atransflective liquid crystal display, a reflective liquid crystaldisplay, a direct-view liquid crystal display, or a projection liquidcrystal display). Display devices having electronic ink, electronicliquid powder, or electrophoretic elements include electronic paper andthe like. In the case of a transflective liquid crystal display or areflective liquid crystal display, some of or all of pixel electrodesfunction as reflective electrodes. For example, some or all of pixelelectrodes are formed to contain aluminum, silver, or the like. In sucha case, a memory circuit such as an SRAM can be provided under thereflective electrodes, leading to lower power consumption.

Each of the plurality of pixel circuits 501 in FIG. 17A can have astructure illustrated in FIG. 17B, for example.

The pixel circuit 501 illustrated in FIG. 17B includes the liquidcrystal element 570, the transistor 550, and the capacitor 560.

The potential of one of a pair of electrodes of the liquid crystalelement 570 is set according to the specifications of the pixel circuit501 as appropriate. The alignment state of the liquid crystal element570 depends on written data. A common potential may be applied to one ofthe pair of electrodes of the liquid crystal element 570 included ineach of the plurality of pixel circuits 501. Further, the potentialsupplied to one of a pair of electrodes of the liquid crystal element570 in the pixel circuit 501 in one row may be different from thepotential supplied to one of a pair of electrodes of the liquid crystalelement 570 in the pixel circuit 501 in another row.

As examples of a driving method of the display device including theliquid crystal element 570, any of the following modes can be given: aTN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell(ASM) mode, an optically compensated birefringence (OCB) mode, aferroelectric liquid crystal (FLC) mode, an antiferroelectric liquidcrystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA)mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode,and the like. Other examples of the driving method of the display deviceinclude electrically controlled birefringence (ECB) mode, polymerdispersed liquid crystal (PDLC) mode, polymer network liquid crystal(PNLC) mode, and a guest-host mode. Note that one embodiment of thepresent invention is not limited thereto, and various liquid crystalelements and driving methods can be used.

In the pixel circuit 501 in the m-th row and the n-th column, one of asource and a drain of the transistor 550 is electrically connected tothe data line DL_n, and the other is electrically connected to the otherof the pair of electrodes of the liquid crystal element 570. A gateelectrode of the transistor 550 is electrically connected to a scan lineGL_m. The transistor 550 has a function of controlling whether to writea data signal by being turned on or off.

One of a pair of electrodes of the capacitor 560 is electricallyconnected to a wiring to which a potential is supplied (hereinafter,referred to as a potential supply line VL), and the other iselectrically connected to the other of the pair of electrodes of theliquid crystal element 570. The potential of the potential supply lineVL is set according to the specifications of the pixel circuit 501 asappropriate. The capacitor 560 functions as a storage capacitor forstoring written data.

For example, in the display device including the pixel circuit 501 inFIG. 17B, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a shown in FIG. 17A, whereby the transistors 550 areturned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuit 501 in whichthe data has been written are brought into a holding state. Thisoperation is sequentially performed row by row; thus, an image isdisplayed.

Each of the plurality of pixel circuits 501 in FIG. 17A can have thestructure shown in FIG. 17C, for example.

The pixel circuit 501 shown in FIG. 17C includes transistors 552 and554, a capacitor 562, and a light-emitting element 572.

One of a source electrode and a drain electrode of the transistor 552 iselectrically connected to a wiring to which a data signal is supplied(hereinafter, referred to as a signal line DL_n). A gate electrode ofthe transistor 552 is electrically connected to a wiring to which a gatesignal is supplied (hereinafter, referred to as a scan line GL_m).

The transistor 552 has a function of controlling whether to write a datasignal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electricallyconnected to a wiring to which a potential is supplied (hereinafter,referred to as a potential supply line VL_a), and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for retaining writtendata.

One of a source electrode and a drain electrode of the transistor 554 iselectrically connected to a potential supply line VL_a. A gate electrodeof the transistor 554 is electrically connected to the other of thesource electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 iselectrically connected to a potential supply line VL_b, and the other iselectrically connected to the other of the source electrode and thedrain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element(also referred to as an organic EL element) or the like can be used, forexample. Note that the light-emitting element 572 is not limited to anorganic EL element; an inorganic EL element including an inorganicmaterial may be used.

A high power supply potential VDD is supplied to one of the potentialsupply line VL_a and the potential supply line VL_b, and a low powersupply potential VSS is supplied to the other of the potential supplyline VL_a and the potential supply line VL_b.

For example, in the display device including the pixel circuit 501 inFIG. 17C, the pixel circuits 501 are sequentially selected row by row bythe gate driver 504 a shown in FIG. 17A, whereby the transistors 552 areturned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in whichthe data has been written are brought into a holding state. Further, theamount of current flowing between the source electrode and the drainelectrode of the transistor 554 is controlled in accordance with thepotential of the written data signal. The light-emitting element 572emits light with a luminance corresponding to the amount of flowingcurrent. This operation is sequentially performed row by row; thus, animage is displayed.

For example, in this specification and the like, a transistor can employa variety of structures. There is no limitation on the type oftransistors. For example, a transistor including single-crystal siliconor a transistor including a non-single-crystal semiconductor filmtypified by amorphous silicon, polycrystalline silicon, microcrystalline(also referred to as microcrystal, nanocrystal, or semi-amorphous)silicon, or the like can be used as a transistor. Alternatively, a thinfilm transistor (TFT) whose semiconductor film is thinned can be used.In the case of using the TFT, there are various advantages. For example,since the TFT can be formed at temperature lower than that of the caseof using single crystalline silicon, a reduction in manufacturing costor an increase in size of a manufacturing apparatus can be achieved.Since the manufacturing apparatus is made larger, the TFT can be formedusing a large substrate. Therefore, many display devices can be formedat the same time at low cost. In addition, a substrate having low heatresistance can be used because of low manufacturing temperature.Therefore, the transistor can be formed using a light-transmittingsubstrate. Alternatively, transmission of light in a display element canbe controlled by using the transistor formed using thelight-transmitting substrate. Alternatively, part of a film included inthe transistor can transmit light because the thickness of thetransistor is small. Therefore, the aperture ratio can be improved.

Note that when a catalyst (e.g., nickel) is used in the case of formingpolycrystalline silicon, crystallinity can be further improved and atransistor having excellent electric characteristics can be formed.Accordingly, a gate driver circuit (e.g., scan line driver circuit), asource driver circuit (e.g., signal line driver circuit), and a signalprocessing circuit (e.g., signal generation circuit, gamma correctioncircuit, or DA converter circuit) can be formed over the same substrateas a pixel portion.

Note that when a catalyst (e.g., nickel) is used in the case of formingmicrocrystalline silicon, crystallinity can be further improved and atransistor having excellent electric characteristics can be formed. Inthis case, crystallinity can be improved by just performing heattreatment without performing laser irradiation. Accordingly, a gatedriver circuit (e.g., a scan line driver circuit) and part of a sourcedriver circuit (e.g., an analog switch) can be formed using the samesubstrate as a pixel portion. Note that when laser irradiation forcrystallization is not performed, unevenness in crystallinity of siliconcan be suppressed. Therefore, high-quality images can be displayed. Notethat it is possible to manufacture polycrystalline silicon ormicrocrystalline silicon without a catalyst (e.g., nickel).

Note that it is preferable that crystallinity of silicon be improved topolycrystal, microcrystal, or the like in the whole panel; however, thepresent invention is not limited to this. Crystallinity of silicon maybe improved only in part of the panel. Selective increase incrystallinity can be achieved by selective laser irradiation or thelike. For example, laser light may be emitted only to a peripheraldriver circuit region which is a region excluding pixel, a region suchas a gate driver circuit and a source driver circuit, or part of asource driver circuit (e.g., an analog switch). Accordingly,crystallinity of silicon can be improved only in a region in which acircuit needs to be operated at high speed. Since a pixel region is notparticularly needed to be operated at high speed, even if crystallinityis not improved, the pixel circuit can be operated without problems.Thus, a region whose crystallinity is improved is small, so thatmanufacturing steps can be decreased. Thus, throughput can be increasedand manufacturing cost can be reduced. Alternatively, since the numberof necessary manufacturing apparatuses is small, manufacturing cost canbe reduced.

Note that for example, a transistor including a compound semiconductor(e.g., SiGe, GaAs, and the like), an oxide semiconductor (e.g., Zn—O,In—Ga—Zn—O, In—Zn—O (indium zinc oxide), In—Sn—O, Sn—O, Ti—O, andAl—Zn—Sn—O, In—Sn—Zn—O), or the like can be used as a transistor.Alternatively, a thin film transistor obtained by thinning any of thecompound semiconductors or the oxide semiconductors cam be used. Sincethe manufacturing temperature can be lowered, such a transistor can beformed at room temperature, for example. Accordingly, the transistor canbe formed directly on a substrate having low heat resistance, such as aplastic substrate or a film substrate. Note that such a compoundsemiconductor or an oxide semiconductor can be used for not only achannel portion of the transistor but also other applications. Forexample, such a compound semiconductor or an oxide semiconductor can beused for a wiring, a resistor, a pixel electrode, a light-transmittingelectrode, or the like. Since such an element can be deposited or formedat the same time as the transistor, cost can be reduced.

Note that for example, a transistor or the like formed by an inkjetmethod or a printing method can be used as a transistor. Accordingly, atransistor can be formed at room temperature, can be formed at a lowvacuum, or can be formed using a large substrate. Therefore, thetransistor can be formed without use of a mask (reticle), so that thelayout of the transistor can be easily changed. Alternatively, since thetransistor can be formed without use of a resist, the material cost isreduced and the number of steps can be reduced. Further, since a filmcan be formed where needed, a material is not wasted as compared to amanufacturing method by which etching is performed after the film isformed over the entire surface; thus, costs can be reduced.

Note that for example, a transistor or the like including an organicsemiconductor or a carbon nanotube can be used as a transistor.Accordingly, such a transistor can be formed using a substrate which canbe bent. A device including a transistor which includes an organicsemiconductor or a carbon nanotube can resist a shock.

Note that transistors with a variety of different structures can be usedas a transistor. For example, a MOS transistor, a junction transistor, abipolar transistor, or the like can be used as a transistor. By using aMOS transistor as a transistor, the size of the transistor can bereduced. Thus, a large number of transistors can be mounted. With use ofa bipolar transistor as a transistor, a large amount of current canflow. Thus, a circuit can be operated at high speed. Note that a MOStransistor and a bipolar transistor may be formed over one substrate.Thus, a reduction in power consumption, reduction in size, high speedoperation, and the like can be achieved.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 4

In this embodiment, a display module which can be formed using a displaydevice of one embodiment of the present invention will be described.

In a display module 8000 in FIG. 18, a touch panel 8004 connected to anFPC 8003, a display panel 8006 connected to an FPC 8005, a backlightunit 8007, a frame 8009, a printed board 8010, and a battery 8011 areprovided between an upper cover 8001 and a lower cover 8002.

The display device of one embodiment of the present invention can beused for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002can be changed as appropriate in accordance with the sizes of the touchpanel 8004 and the display panel 8006. The upper cover 8001 and thelower cover 8002 may have flexibility.

The touch panel 8004 can be a resistive touch panel or a capacitivetouch panel and may be formed so as to overlap with the display panel8006. A counter substrate (sealing substrate) of the display panel 8006can have a touch panel function. A photosensor may be provided in eachpixel of the display panel 8006 so that the touch panel 8004 canfunction as an optical touch panel. The touch panel 8004 may haveflexibility.

The backlight unit 8007 includes a light source 8008. Note that althougha structure in which the light sources 8008 are provided over thebacklight unit 8007 is illustrated in FIG. 18, one embodiment of thepresent invention is not limited to this structure. For example, astructure in which a light source 8008 is provided at an end portion ofthe backlight unit 8007 and a light diffusion plate is further providedmay be employed. In the case where a self-luminous light-emittingelement such as an organic EL element is used or the case where areflective panel is used, the backlight unit 8007 is not necessarilyprovided. The backlight unit 8007 may have flexibility.

The frame 8009 protects the display panel 8006 and functions as anelectromagnetic shield for blocking electromagnetic waves generated bythe operation of the printed board 8010. The frame 8009 can function asa radiator plate. The frame 8009 may have flexibility.

The printed board 8010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying power to the power supply circuit, anexternal commercial power source or a power source using the battery8011 provided separately may be used. The battery 8011 can be omitted inthe case of using a commercial power source. The printed board 8010 maybe an FPC.

The display module 8000 may be additionally provided with a member suchas a polarizing plate, a retardation plate, or a prism sheet.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 5

In this embodiment, a transistor that can be used for a display deviceof one embodiment of the present invention and a material included inthe transistor will be described. The transistor described in thisembodiment can be used for the transistors 350, 352, 550, 552, 554, andthe like described in the above embodiment. Note that the transistordescribed in this embodiment has a structure in a state before beingtransferred to a flexible substrate.

FIG. 19A is a cross-sectional view of an example of a transistor thatcan be used in a display device of one embodiment of the presentinvention. The transistor includes an organic resin layer 910 over asubstrate 900, an insulating film 915, a gate electrode layer 920, agate insulating film 930 in which an gate insulating film 931 and angate insulating film 932 are stacked in this order, an oxidesemiconductor layer 940, and a source electrode layer 950 and a drainelectrode layer 960 in contact with part of the oxide semiconductorlayer. In addition, an insulating film 970, an insulating film 980, andan insulating film 990 may be formed over the gate insulating film 930,the oxide semiconductor layer 940, the source electrode layer 950, andthe drain electrode layer 960.

The transistor of one embodiment of the present invention may include,as illustrated in FIG. 19B, a conductive film 921 that overlaps with thegate electrode layer 920 and the oxide semiconductor layer 940 and isover the insulating film 980 or the insulating film 990. When theconductive film is used as a second gate electrode layer (back gate),the on-state current can be increased and the threshold voltage can becontrolled. To increase the on-state current, for example, the gateelectrode layer 920 and the conductive film 921 are set to have the samepotential, and the transistor is driven as a dual-gate transistor.Further, to control the threshold voltage, a fixed potential that isdifferent from a potential of the gate electrode layer 920 is suppliedto the conductive film 921.

The transistor of one embodiment of the present invention may have achannel-protective bottom-gate structure as illustrated in FIGS. 25A and25B. In this structure, an insulating film 933 has a function ofprotecting a channel region. Thus, the insulating film 933 may beprovided only in a region overlapping with the channel region orprovided in a region besides the region as illustrated in FIGS. 25A and25B.

The transistor of one embodiment of the present invention may have aself-aligned top-gate structure as illustrated in FIGS. 26A and 26B. Inthe structure in FIG. 26A, a source region 951 and a drain region 961can be formed in the following manner: oxygen vacancies are generated bymaking the source electrode layer 950 and the drain electrode layer 960being in contact with an oxide semiconductor layer; or the oxidesemiconductor layer is doped with impurities such as boron, phosphorus,or argon using the gate electrode layer 920 as a mask. In the structurein FIG. 26B, the source region 951 and the drain region 961 can beformed, instead of using the doping method, in the following manner: aninsulating film 975 containing hydrogen, such as a silicon nitride film,is formed to be in contact with part of the oxide semiconductor layer940 and the hydrogen is diffused to the part of the oxide semiconductorlayer 940.

The transistor of one embodiment of the present invention may have aself-aligned top-gate structure as illustrated in FIG. 27A. In thestructure in FIG. 27A, the source region 951 and the drain region 961can be formed in the following manner: oxygen vacancies are generated bymaking the source electrode layer 950 and the drain electrode layer 960being in contact with an oxide semiconductor layer; or the oxidesemiconductor layer is doped with impurities such as boron, phosphorus,or argon using the gate insulating film 930 as a mask. In the structurein FIG. 27A, the source electrode layer 950, the drain electrode layer960, and the gate electrode layer 920 can be formed in one process.

The transistor of one embodiment of the present invention may have aself-aligned top-gate structure as illustrated in FIG. 27B. In thestructure in FIG. 27B, the source region 951 and the drain region 961can be formed, besides using the doping method with impurities such asboron, phosphorus, or argon using the gate insulating film 930 as amask, in the following manner: the insulating film 975 containinghydrogen, such as a silicon nitride film, is formed to be in contactwith part of the oxide semiconductor layer 940 and the hydrogen isdiffused to the part of the oxide semiconductor layer 940. In thestructure, the source region 951 and the drain region 961 can have lowerresistance. Alternatively, a structure in which doping with theimpurities is not performed or a structure without the insulating film975 can be formed.

Note that elements which form oxygen vacancies in the oxidesemiconductor layer are described as impurities (impurity elements).Typical examples of impurity elements are boron, carbon, nitrogen,fluorine, aluminum, silicon, phosphorus, chlorine, and rare gaselements. Typical examples of rare gas elements are helium, neon, argon,krypton, and xenon.

When hydrogen is added to an oxide semiconductor in which an oxygenvacancy is generated by addition of an impurity element, hydrogen entersan oxygen vacant site and forms a donor level in the vicinity of theconduction band. As a result, the conductivity of the oxidesemiconductor is increased, so that the oxide semiconductor becomes aconductor. An oxide semiconductor having become a conductor can bereferred to as an oxide conductor. Oxide semiconductors generally have avisible light transmitting property because of their large energy gap.An oxide conductor is an oxide semiconductor having a donor level in thevicinity of the conduction band. Therefore, the influence of absorptiondue to the donor level is small, and an oxide conductor has a visiblelight transmitting property comparable to that of an oxidesemiconductor.

The temperature dependence of resistivity in a film formed using anoxide conductor (hereinafter, referred to as oxide conductive layer) isdescribed with reference to FIG. 36.

Here, a sample including an oxide conductive layer was formed. As theoxide conductive layer, the following oxide conductive layers wereformed: an oxide conductive layer (OC_SiN_(x)) formed by making asilicon nitride film being in contact with an oxide semiconductor layer;an oxide conductive layer (OC_Ar dope+SiN_(x)) formed by adding argon toan oxide semiconductor layer with an doping apparatus and making theoxide semiconductor layer being in contact with a silicon nitride film;and an oxide conductive layer (OC_Ar plasma+SiN_(x)) formed by exposingan oxide semiconductor layer to argon plasma with a plasma treatmentapparatus and making the oxide semiconductor layer being in contact witha silicon nitride film. The silicon nitride film contains hydrogen.

A method for forming a sample including the oxide conductive layer(OC_SiN_(x)) is described below. A 400-nm-thick silicon oxynitride filmwas deposited over a glass substrate by a plasma CVD method and thenexposed to oxygen plasma so that an oxygen ion was added to the siliconoxynitride film, whereby an oxynitride silicon film from which oxygen isreleased by heating was formed. Next, over the oxynitride silicon filmfrom which oxygen is released by heating, a 100-nm-thick In—Ga—Zn oxidefilm was deposited by a sputtering method using a sputtering target withan atomic ratio In:Ga:Zn=5:5:6, subjected to heat treatment in anitrogen atmosphere at 450° C., and then subjected to heat treatment ina mixed gas of nitrogen and oxygen at 450° C. Next, a 100-nm-thicksilicon nitride film was deposited by a plasma CVD method. Then, thefilm was subjected to heat treatment in a mixed gas of nitrogen andoxygen at 350° C.

A method for forming a sample including the oxide conductive layer(OC_Ar dope+SiN_(x)) is described below. A 400-nm-thick siliconoxynitride film was deposited over a glass substrate by a plasma CVDmethod and then exposed to oxygen plasma so that an oxygen ion was addedto the silicon oxynitride film, whereby an oxynitride silicon film fromwhich oxygen is released by heating was formed. Next, over theoxynitride silicon film from which oxygen is released by heating, a100-nm-thick In—Ga—Zn oxide film was deposited by a sputtering methodusing a sputtering target with an atomic ratio In:Ga:Zn=5:5:6, subjectedto heat treatment in a nitrogen atmosphere at 450° C., and thensubjected to heat treatment in a mixed gas of nitrogen and oxygen at450° C. Next, with a doping apparatus, argon with a dose of 5×10¹⁴/cm²was added to the In—Ga—Zn oxide film at an accelerating voltage of 10kV, whereby an oxygen vacancy was formed in the In—Ga—Zn oxide film.Next, a 100-nm-thick silicon nitride film was deposited by a plasma CVDmethod. Then, the film was subjected to heat treatment in a mixed gas ofnitrogen and oxygen at 350° C.

A method for forming a sample including the oxide conductive layer(OC_Ar plasma+SiN_(x)) is described below. A 400-nm-thick siliconoxynitride film was deposited over a glass substrate by a plasma CVDmethod and then exposed to oxygen plasma, whereby an oxynitride siliconfilm from which oxygen is released by heating was formed. Next, over theoxynitride silicon film from which oxygen is released by heating, a100-nm-thick In—Ga—Zn oxide film was deposited by a sputtering methodusing a sputtering target with an atomic ratio In:Ga:Zn=5:5:6, subjectedto heat treatment in a nitrogen atmosphere at 450° C., and thensubjected to heat treatment in a mixed gas of nitrogen and oxygen at450° C. Next, argon plasma was generated with a plasma treatmentapparatus, and an accelerated argon ion was made to collide against theIn—Ga—Zn oxide film, whereby an oxygen vacancy was generated. Next, a100-nm-thick silicon nitride film was deposited by a plasma CVD method.Then, the film was subjected to heat treatment in a mixed gas ofnitrogen and oxygen at 350° C.

FIG. 36 shows the measured resistivity of each sample. The measurementof resistivity was performed by the four probe Van der Pauw method. InFIG. 36, the horizontal axis represents measurement temperature, and thevertical axis represents resistivity. Squares, triangles, and circlesindicates the measurement results of oxide conductive layer(OC_SiN_(x)), the measurement results of oxide conductive layer (OC_Ardope+SiN_(x)), and the measurement results of oxide conductive layer(OC_Ar plasma+SiN_(x)), respectively.

The oxide semiconductor layer that is not in contact with the siliconnitride film, though not shown, had high resistivity, and it wasdifficult to measure the resistivity. Thus, it is found that the oxideconductive layer has lower resistivity than the oxide semiconductorlayer.

According to FIG. 36, in the case where the oxide conductive layer(OC_Ar dope+SiN_(x)) and the oxide conductive layer (OC_Arplasma+SiN_(x)) contain oxygen vacancy and hydrogen, a variation inresistivity is small. Typically, the variation in resistivity attemperatures from 80 K to 290 K is lower than ±20%. Alternatively, thevariation in resistivity at temperatures from 150 K to 250 K is lowerthan ±10%. In other words, the oxide conductor is a degeneratesemiconductor and it is suggested that the conduction band edge agreeswith or substantially agrees with the Fermi level. Thus, when the oxideconductive layer is used as a source region and a drain region of atransistor, an ohmic contact occurs at a portion where the oxideconductive layer is in contact with a conductive film functioning as asource electrode and a drain electrode, and the contact resistance ofthe oxide conductive layer and the conductive film functioning as asource electrode and a drain electrode can be reduced. Furthermore, theoxide conductor has low temperature resistance of resistivity; thus, afluctuation of contact resistance of the oxide semiconductor layer and aconductive film functioning as a source electrode and a drain electrodeis small, and a highly reliable transistor can be formed.

The transistor of one embodiment of the present invention may include aconductive film 921 overlapping with the oxide semiconductor layer 940with the gate insulating film 935 interposed therebetween as illustratedin FIGS. 28A and 28B. Although FIGS. 28A and 28B illustrate exampleswhere the conductive film 921 is provided in the transistors illustratedin FIGS. 26A and 26B, the conductive film 921 can be provided in thetransistors illustrated in FIGS. 27A and 27B.

In the display device of one embodiment of the present invention, anoxide semiconductor is used in an active layer as described above. Thetransistor using an oxide semiconductor layer has a higher mobility thana transistor using amorphous silicon, and is thus easily reduced insize, resulting in a reduction in the size of a pixel. The transistorusing an oxide semiconductor layer enables a flexible display device tohave high reliability. Note that an embodiment of the present inventionis not limited thereto. An active layer may include a semiconductorother than an oxide semiconductor depending on the case or condition.

Note that as illustrated in FIGS. 19A and 19B and the like, the width ofthe gate electrode layer 920 is preferably larger than that of the oxidesemiconductor layer 940. In the display device having a backlight, thegate electrode layer functions as a light-blocking layer, and adeterioration of electric characteristics, caused by irradiation of theoxide semiconductor layer 940 with light, can be suppressed. In an ELdisplay device, a gate electrode in a top-gate transistor can functionas a light-blocking layer.

Next, the components of the transistor of one embodiment of the presentinvention will be described in detail.

The substrate 900 is preferably a rigid substrate because a step oftransferring the component to a flexible substrate is easily performed.For example, a glass substrate, a ceramic substrate, a quartz substrate,a sapphire substrate, a metal substrate, or the like can be used. Notethat the substrate 900 corresponds to the first substrate 462 inEmbodiment 1.

As the organic resin layer 910, for example, an organic resin such as anepoxy resin, an aramid resin, an acrylic resin, a polyimide resin, apolyamide resin, or a polyamide-imide resin can be used. Note that theorganic resin layer 910 corresponds to the organic resin layer 320 a inEmbodiment 1.

As the insulating film 915, for example, a single layer such as asilicon oxide film, a silicon oxynitride film, a silicon nitride film,or a silicon nitride oxide film, or a stacked film including any of theabove films can be used. The insulating film 915 corresponds to thefirst insulating film 321 a in Embodiment 1.

The gate electrode layer 920 and the conductive film 921 can be formedusing a metal element selected from chromium (Cr), copper (Cu), aluminum(Al), gold (Au), silver (Ag), zinc (Zn), molybdenum (Mo), tantalum (Ta),titanium (Ti), tungsten (W), manganese (Mn), nickel (Ni), iron (Fe), orcobalt (Co), an alloy including the above metal element, an alloy inwhich any of the above metal elements are combined, or the like.Furthermore, the gate electrode layer 920 may have a single-layerstructure or a stacked structure of two or more layers.

Alternatively, the gate electrode layer 920 and the conductive film 921can be formed using a light-transmitting conductive material such asindium tin oxide, indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium zinc oxide, or indiumtin oxide to which silicon oxide is added. It is also possible to have alayered structure formed using the above light-transmitting conductivematerial and the above metal element.

Further, an In—Ga—Zn-based oxynitride semiconductor film, an In—Sn-basedoxynitride semiconductor film, an In—Ga-based oxynitride semiconductorfilm, an In—Zn-based oxynitride semiconductor film, a Sn-basedoxynitride semiconductor film, an In-based oxynitride semiconductorfilm, a film of metal nitride (such as InN or ZnN), or the like may beprovided between the gate electrode layer 920 and the gate insulatingfilm 932.

As each of the gate insulating films 931 and 932 that are the gateinsulating film 930, an insulating layer including at least one of thefollowing films formed by a plasma enhanced chemical vapor deposition(PECVD) method, a sputtering method, or the like can be used: a siliconoxide film, a silicon oxynitride film, a silicon nitride oxide film, asilicon nitride film, an aluminum oxide film, a hafnium oxide film, anyttrium oxide film, a zirconium oxide film, a gallium oxide film, atantalum oxide film, a magnesium oxide film, a lanthanum oxide film, acerium oxide film, and a neodymium oxide film. Note that instead of astacked structure of the gate insulating films 931 and 932, the gateinsulating film 930 may be an insulating film of a single layer formedusing a material selected from the above or an insulating film of threeor more layers.

Note that the gate insulating film 932 that is in contact with the oxidesemiconductor layer 940 functioning as a channel formation region of thetransistor is preferably an oxide insulating film and preferably has aregion (oxygen-excess region) containing oxygen in excess of thestoichiometric composition. In other words, the gate insulating film 932is an insulating film from which oxygen can be released. In order toprovide the oxygen-excess region in the gate insulating film 932, thegate insulating film 932 is formed in an oxygen atmosphere, for example.Alternatively, oxygen may be introduced into the deposited gateinsulating film 932 to provide the oxygen-excess region therein. Oxygencan be introduced by an ion implantation method, an ion doping method, aplasma immersion ion implantation method, plasma treatment, or the like.

In the case where hafnium oxide is used for the gate insulating films931 and 932, the following effect is attained. Hafnium oxide has higherdielectric constant than silicon oxide and silicon oxynitride.Therefore, by using hafnium oxide or aluminum oxide, a physicalthickness can be made larger than an equivalent oxide thickness; thus,even in the case where the equivalent oxide thickness is less than orequal to 10 nm or less than or equal to 5 nm, leakage current due totunnel current can be low. That is, it is possible to provide atransistor with a low off-state current. Moreover, hafnium oxide with acrystalline structure has higher dielectric constant than hafnium oxidewith an amorphous structure. Therefore, it is preferable to use hafniumoxide with a crystalline structure in order to provide a transistor witha low off-state current. Examples of the crystalline structure include amonoclinic crystal structure and a cubic crystal structure. Note thatone embodiment of the present invention is not limited to the aboveexamples.

In this embodiment, a silicon nitride film is formed as the insulatingfilm 931, and a silicon oxide film is formed as the gate insulating film932. In addition, a silicon nitride film has a higher dielectricconstant than a silicon oxide film and needs a larger thickness forcapacitance equivalent to that of the silicon oxide. Thus, when asilicon nitride film is used for the gate insulating film 930 of thetransistor, the physical thickness of the gate insulating film can beincreased. From the above, the electrostatic breakdown of the transistorcan be prevented by inhibiting a reduction in the withstand voltage ofthe transistor and further improving the withstand voltage of thetransistor.

The oxide semiconductor layer 940 is typically formed using an In—Gaoxide, an In—Zn oxide, or an In-M-Zn oxide (M is Al, Ti, Ga, Y, Zr, La,Ce, Nd, Sn, or Hf). In particular, an In-M-Zn oxide (M is Al, Ti, Ga, Y,Zr, La, Ce, Nd, Sn, or Hf) is preferably used for the oxidesemiconductor layer 940.

In the case where the oxide semiconductor layer 940 is an In-M-Zn oxide(M is Al, Ti, Ga, Y, Zr, La, Ce, Nd, Sn, or Hf), it is preferable thatthe atomic ratio of metal elements of a sputtering target used forforming a film of the In-M-Zn oxide satisfy In≥M and Zn≥M. As the atomicratio of metal elements of such a sputtering target, In:M:Zn=1:1:1,In:M:Zn=5:5:6, and In:M:Zn=3:1:2 are preferable. Note that the atomicratio of metal elements in the formed oxide semiconductor layer 940varies from the above atomic ratio of metal elements of the sputteringtarget within a range of ±40% as an error.

In the case of using an In-M-Zn oxide for the oxide semiconductor layer940, when Zn and O are eliminated from consideration, the proportion ofIn and the proportion of M are preferably greater than or equal to 25atomic % and less than 75 atomic %, respectively, further preferablygreater than or equal to 34 atomic % and less than 66 atomic %,respectively.

The energy gap of the oxide semiconductor layer 940 is 2 eV or more,preferably 2.5 eV or more, and more preferably 3 eV or more. In thismanner, the amount of off-state current of a transistor can be reducedby using an oxide semiconductor having a wide energy gap.

The oxide semiconductor layer 940 has a thickness greater than or equalto 3 nm and less than or equal to 200 nm, preferably 3 nm to 100 nm, andfurther preferably 3 nm to 50 nm.

An oxide semiconductor layer with low carrier density is used as theoxide semiconductor layer 940. For example, an oxide semiconductor layerwhose carrier density is lower than or equal to 1×10¹⁷/cm³, preferablylower than or equal to 1×10¹⁵/cm³, further preferably lower than orequal to 1×10¹³/cm³, still further preferably lower than or equal to1×10¹¹/cm³ is used as the oxide semiconductor layer 940.

However, the composition is not limited to those described above, and amaterial having the appropriate composition may be used depending onrequired semiconductor characteristics and electrical characteristics ofthe transistor (e.g., field-effect mobility and threshold voltage).Further, in order to obtain the required semiconductor characteristicsof the transistor, it is preferable that the carrier density, theimpurity concentration, the defect density, the atomic ratio of a metalelement to oxygen, the interatomic distance, the density, and the likeof the oxide semiconductor layer 940 be set to appropriate values.

Further, in the oxide semiconductor layer, hydrogen, nitrogen, carbon,silicon, and metal elements except for main components are impurities.For example, hydrogen and nitrogen form donor levels to increase thecarrier density. Silicon forms impurity levels in the oxidesemiconductor layer. The impurity level becomes a trap, which mightdeteriorate the electric characteristics of the transistor. It ispreferable to reduce the concentration of the impurities in the oxidesemiconductor layer and at interfaces with other layers.

Note that stable electrical characteristics can be effectively impartedto a transistor in which an oxide semiconductor layer serves as achannel by reducing the concentration of impurities in the oxidesemiconductor layer to make the oxide semiconductor layer intrinsic orsubstantially intrinsic. The term “substantially intrinsic” refers tothe state where an oxide semiconductor layer has a carrier density whichis lower than 1×10¹⁷/cm³, preferably lower than 1×10¹⁵/cm³, furtherpreferably lower than 1×10¹³/cm³, particularly preferably lower than8×10¹¹/cm³, still further preferably lower than 1×10¹¹/cm³, yet furtherpreferably lower than 1×10¹⁰/cm³, and is 1×10⁻⁹/cm³ or higher.

In order to make the oxide semiconductor layer intrinsic orsubstantially intrinsic, in SIMS (secondary ion mass spectrometry), forexample, the concentration of silicon at a certain depth of the oxidesemiconductor layer or in a region of the oxide semiconductor layer islower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸ atoms/cm³,more preferably lower than 1×10¹⁸ atoms/cm³. Further, the concentrationof hydrogen at a certain depth of the oxide semiconductor layer or in aregion of the oxide semiconductor layer is lower than or equal to 2×10²⁰atoms/cm³, preferably lower than or equal to 5×10¹⁹ atoms/cm³, furtherpreferably lower than or equal to 1×10¹⁹ atoms/cm³, still furtherpreferably lower than or equal to 5×10¹⁸ atoms/cm³. Further, theconcentration of nitrogen at a certain depth of the oxide semiconductorlayer or in a region of the oxide semiconductor layer is lower than5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸ atoms/cm³,further preferably lower than or equal to 1×10¹⁸ atoms/cm³, stillfurther preferably lower than or equal to 5×10¹⁷ atoms/cm³.

In the case where the oxide semiconductor layer includes crystals, highconcentration of silicon or carbon might reduce the crystallinity of theoxide semiconductor layer. In order not to lower the crystallinity ofthe oxide semiconductor layer, for example, the concentration of siliconat a certain depth of the oxide semiconductor layer or in a region ofthe oxide semiconductor layer is lower than 1×10¹⁹ atoms/cm³, preferablylower than 5×10¹⁸ atoms/cm³, further preferably lower than 1×10¹⁸atoms/cm³. Further, the concentration of carbon at a certain depth ofthe oxide semiconductor layer or in a region of the oxide semiconductorlayer is lower than 1×10¹⁹ atoms/cm³, preferably lower than 5×10¹⁸atoms/cm³, further preferably lower than 1×10¹⁸ atoms/cm³, for example.

Various experiments can prove low off-state current of a transistorincluding a highly purified oxide semiconductor layer for a channelformation region. For example, even when an element has a channel widthof 1×10⁶ μm and a channel length of 10 μm, off-state current can be lessthan or equal to the measurement limit of a semiconductor parameteranalyzer, i.e., less than or equal to 1×10⁻¹³ A, at voltage (drainvoltage) between the source electrode and the drain electrode of from 1V to 10 V. In this case, it can be seen that the off-state currentnormalized on the channel width of the transistor is lower than or equalto 100 zA/μm. In addition, a capacitor and a transistor are connected toeach other and the off-state current is measured with a circuit in whichcharge flowing into or from the capacitor is controlled by thetransistor. In the measurement, a highly purified oxide semiconductorlayer is used for a channel formation region of the transistor, and theoff-state current density of the transistor is measured by a change inthe amount of electric charge of the capacitor per unit time. As aresult, it is found that in the case where the voltage between thesource electrode and the drain electrode of the transistor is 3 V, loweroff-state current of several tens of yoctoamperes per micrometer (yA/μm)can be obtained. Accordingly, the off-state current of the transistorincluding a channel formation region formed of the highly purified oxidesemiconductor layer is considerably lower than that of a transistorincluding silicon having crystallinity.

For the source electrode layer 950 and the drain electrode layer 960, aconductive film having properties of extracting oxygen from the oxidesemiconductor layer is preferably used. For example, Al, Cr, Cu, Ta, Ti,Mo, W, Ni, Mn, Nd, or Sc can be used. It is also possible to use analloy or a conductive nitride of any of these materials. It is alsopossible to use a stack of a plurality of materials selected from thesematerials, alloys of these materials, and conductive nitrides of thesematerials. Typically, it is preferable to use Ti, which is particularlyeasily bonded to oxygen, or W, which has a high melting point and thusallows subsequent process temperatures to be relatively high.Alternatively, Cu or a Cu—X alloy (X indicates Mn, Ni, Cr, Fe, Co, Mo,Ta, or Ti), which has low resistance may be used. Further alternatively,a stacked layer including any of the above materials and Cu or Cu— Xalloy may be used.

In the case of using Cu—X alloy (X indicates Mn, Ni, Cr, Fe, Co, Mo, Ta,or Ti), a covering film is formed in a region in contact with the oxidesemiconductor layer or a region in contact with an insulating film byheat treatment, in some cases. The covering layer includes a compoundcontaining X Examples of compound containing X include an oxide of X, anIn—X oxide, a Ga—X oxide, an In—Ga—X oxide, and In—Ga—Zn—X oxide. Whenthe covering film is formed, the covering film functions as a blockingfilm, and Cu in the Cu—X alloy film can be prevented from entering theoxide semiconductor layer.

By the conductive film capable of extracting oxygen from the oxidesemiconductor layer, oxygen in the oxide semiconductor layer is releasedto form oxygen vacancies in the oxide semiconductor film. Hydrogenslightly contained in the layer and the oxygen vacancy are bonded toeach other, whereby the region is markedly changed to an n-type region.Accordingly, the n-type regions can serve as a source or a drain regionof the transistor.

The insulating films 970, 980, and 990 each have a function of aprotective insulating film. For example, the insulating film 970 is aninsulating film through which oxygen can be transmitted. In addition,the insulating film 970 also has a function of lessening the damage tothe oxide semiconductor layer 940 at the time of forming the insulatingfilm 980.

As the oxide insulating film 970, a silicon oxide film, a siliconoxynitride film, or the like having a thickness greater than or equal to5 nm and less than or equal to 150 nm, preferably greater than or equalto 5 nm and less than or equal to 50 nm can be used. Note that in thisspecification, “silicon oxynitride film” refers to a film that containsmore oxygen than nitrogen, and “silicon nitride oxide film” refers to afilm that contains more nitrogen than oxygen.

Further, it is preferable that the number of defects in the insulatingfilm 970 be small and typically, the spin density of a signal thatappears at g=2.001 due to a dangling bond of silicon be lower than orequal to 3×10¹⁷ spins/cm³ by electron spin resonance (ESR) measurement.This is because if the density of defects in the insulating film 970 ishigh, oxygen is bonded to the defects and the amount of oxygen thatpasses through the insulating film 970 is decreased.

The insulating film 980 is formed using an oxide insulating film whoseoxygen content is in excess of that in the stoichiometric composition.Part of oxygen is released by heating from the oxide insulating filmcontaining oxygen content than that in the stoichiometric composition.The oxide insulating film containing oxygen content than that in thestoichiometric composition is an oxide insulating film of which theamount of released oxygen converted into oxygen atoms is greater than orequal to 1.0×10¹⁸ atoms/cm³, preferably greater than or equal to3.0×10²⁰ atoms/cm³ in thermal desorption spectroscopy (TDS) analysis inwhich heat treatment is performed such that a temperature of a filmsurface is higher than or equal to 100° C. and lower than or equal to700° C. or higher than or equal to 100° C. and lower than or equal to500° C.

A silicon oxide film, a silicon oxynitride film, or the like with athickness greater than or equal to 30 nm and less than or equal to 500nm, preferably greater than or equal to 50 nm and less than or equal to400 nm can be used for the insulating film 980.

Further, it is preferable that the amount of defects in the insulatingfilm 980 be small, typically the spin density of a signal which appearsat g=2.001 originating from a dangling bond of silicon, be lower than1.5×10¹⁸ spins/cm³, further preferably lower than or equal to 1×10¹⁸spins/cm³ by ESR measurement. Note that the insulating film 980 isprovided more apart from the oxide semiconductor layer 940 than theinsulating film 970 is; thus, the insulating film 980 may have higherdefect density than the insulating film 970.

Furthermore, since the insulating films 970 and 980 can be formed usingthe same kind of material, it may be difficult to clearly distinguishthe boundary between the insulating film 970 and the insulating film980. Thus, in this embodiment, the boundary between the insulating films970 and 980 is shown by a dashed line. Although a two-layer structure ofthe insulating films 970 and 980 is described in this embodiment, thepresent invention is not limited to this. For example, a single-layerstructure of the insulating film 970, a single-layer structure of theinsulating film 980, or a stacked-layer structure including three ormore layers may be used.

The insulating film 990 can have a function of blocking oxygen,hydrogen, water, an alkali metal, an alkaline earth metal, or the like.With the insulating film 990, oxygen diffusion from the oxidesemiconductor layer 940 to the outside and entry of hydrogen, water, orthe like from the outside to the oxide semiconductor layer 940 can beprevented. As the insulating film 990, a nitride insulating film can beused, for example. The nitride insulating film is formed using siliconnitride, silicon nitride oxide, aluminum nitride, aluminum nitrideoxide, or the like. Note that instead of the nitride insulating filmhaving a blocking effect against oxygen, hydrogen, water, alkali metal,alkaline earth metal, and the like, an oxide insulating film having ablocking effect against oxygen, hydrogen, water, and the like, may beprovided. As the oxide insulating film having a blocking effect againstoxygen, hydrogen, water, and the like, an aluminum oxide film, analuminum oxynitride film, a gallium oxide film, a gallium oxynitridefilm, an yttrium oxide film, an yttrium oxynitride film, a hafnium oxidefilm, and a hafnium oxynitride film can be given.

Note that the oxide semiconductor layer 940 may have a structure inwhich a plurality of oxide semiconductor layers are stacked. Forexample, as in a transistor illustrated in FIG. 20A, stacked layers of afirst oxide semiconductor layer 941 a and a second oxide semiconductorlayer 941 b may constitute the oxide semiconductor layer 940. The firstoxide semiconductor layer 941 a and the second oxide semiconductor layer941 b may include metal oxides having different atomic ratios. Forexample, one of the oxide semiconductor layers may include one of anoxide containing two kinds of metals, an oxide containing three kinds ofmetals, and an oxide containing four kinds of metals, and the other ofthe oxide semiconductor layers may include another one of the oxidecontaining two kinds of metals, the oxide containing three kinds ofmetals, and the oxide containing four kinds of metals.

Alternatively, the first oxide semiconductor layer 941 a and the secondoxide semiconductor layer 941 b may include the same constituentelements with different atomic ratios. For example, one of the oxidesemiconductor layers may contain In, Ga, and Zn at an atomic ratio of1:1:1, 5:5:6, or 3:1:2, and the other of the oxide semiconductor layersmay contain In, Ga, and Zn at an atomic ratio of 1:3:2, 1:3:4, 1:3:6,1:4:5, 1:6:4, or 1:9:6. Note that the atomic ratio of each oxidesemiconductor layer varies within a range of ±20% of the above atomicratio as an error.

In the above, one of the oxide semiconductor layers, which is closer tothe gate electrode (the oxide semiconductor layer on the channel side),has an atomic ratio of In≥Ga (in the atomic ratio, In is greater than orequal to Ga); and the other oxide semiconductor layer, which is fartherfrom the gate electrode (the oxide semiconductor layer on the backchannel side), has an atomic ratio of In<Ga. In that case, a transistorwith a high field-effect mobility can be manufactured. On the otherhand, when the oxide semiconductor layer on the channel side has anatomic ratio of In<Ga and the oxide semiconductor layer on the backchannel side has an atomic ratio of In≥Ga (in the atomic ratio, In isgreater than or equal to Ga), it is possible to reduce the amount ofchange in the threshold voltage of a transistor due to change over timeor a reliability test.

Further alternatively, the semiconductor film of the transistor may havea three-layer structure of a first oxide semiconductor layer, a secondoxide semiconductor layer, and a third oxide semiconductor layer. Inthat case, the first to third oxide semiconductor layers may include thesame constituent elements with different atomic ratios. A transistorincluding a three-layer semiconductor film will be described withreference to FIG. 20B and FIGS. 29A and 29B. Note that a structure inwhich a semiconductor film has a multilayer structure can be employedfor the other transistor described in this embodiment.

Each transistor illustrated in FIG. 20B and FIGS. 29A and 29B includesthe third oxide semiconductor layer 942 a, the second oxidesemiconductor layer 942 b, and the first oxide semiconductor layer 942 cwhich are stacked in this order from the gate insulating film side.

The first oxide semiconductor layer 942 c and the third oxidesemiconductor layer 942 a are formed using a material represented byInM_(1x)Zn_(y)O_(z) (x≥1, y>1, z>0, M₁=Ga, Hf, or the like, where x isgreater than or equal to 1). The second oxide semiconductor layer 942 bis formed using a material which can be represented byInM_(2x)Zn_(y)O_(z) (x≥1, y≥x, z>0, M₂=Ga, Sn, or the like, where x isgreater than or equal to 1 and y is greater than or equal to x).

Materials of the first to third oxide semiconductor layers are selectedas appropriate so as to form a well-shaped structure in which theconduction band minimum in the second oxide semiconductor layer 942 b isdeeper from the vacuum level than the conduction band minimum in thefirst and third oxide semiconductor layers 942 c and 942 a.

For example, the first oxide semiconductor layer 942 c and the thirdoxide semiconductor film 942 a may each have an atomic ratio ofIn:Ga:Zn=1:1:1, 1:3:2, 1:3:4, 1:3:6, 1:4:5, 1:6:4, or 1:9:6; the secondoxide semiconductor layer 942 b may have an atomic ratio ofIn:Ga:Zn=1:1:1, 5:5:6, or 3:1:2.

Since the first to third oxide semiconductor layers 942 c to 942 ainclude the same constituent elements, the second oxide semiconductorlayer 942 b has few defect states (trap levels) at the interface withthe third oxide semiconductor layer 942 a. Specifically, the defectstates (trap levels) are fewer than those at the interface between thegate insulating film and the third oxide semiconductor layer 942 a. Forthis reason, when the oxide semiconductor layers are stacked in theabove manner, the amount of change in the threshold voltage of atransistor due to a change over time or a reliability test can bereduced.

Further, materials of the first to third oxide semiconductor layers areselected as appropriate so as to form a well-shaped structure in whichthe conduction band minimum in the second oxide semiconductor layer 942b is deeper from the vacuum level than the conduction band minimum inthe first and third oxide semiconductor layers 942 c and 942 a. As aresult, the field-effect mobility of the transistor can be increased andthe amount of change in the threshold voltage of the transistor due tochange over time or a reliability test can be reduced.

Further, the first to third oxide semiconductor layers 942 c to 942 amay be formed using oxide semiconductors having differentcrystallinities. Note that at least the second oxide semiconductor layer942 b that can function as a channel formation region is preferably afilm with crystallinity, further preferably a film in which c-axes arealigned perpendicularly to a surface.

Structures illustrated in FIGS. 35A and 35B are preferably for a crosssection in the channel width direction, of a channel formation region ina top-gate transistor illustrated in FIG. 29A or the like. In each ofthe above structures, the gate electrode layer 920 electricallysurrounds the oxide semiconductor layer 940 in the channel widthdirection. Such a transistor structure is referred to as a surroundedchannel (s-channel) structure.

In the structure including the conductive film 921 as illustrated inFIGS. 28A and 28B, the gate electrode layer 920 and the conductive film921 may be connected to each other through a contact hole, asillustrated in FIG. 35B, so as to have the same potential.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 6

In this embodiment, a transistor included in a display device of oneembodiment of the present invention will be described.

The transistors included in the display device of one embodiment of thepresent invention do not necessarily have a uniform structure. Forexample, a transistor in a pixel portion in the display device and atransistor used in a driver circuit portion for driving the pixelportion have different structures; thus, the transistors can haveelectric characteristics appropriate to the respective portions, and thereliability of the display device can be improved.

When the transistor included in the driver circuit has a double gatestructure, the transistor has high field-effect mobility.

Furthermore, the transistor in the driver circuit portion and thetransistor in the pixel portion may have different channel lengths.Typically, the channel length of a transistor 194 in the driver circuitportion is less than 2.5 μm, or greater than or equal to 1.45 μm andless than or equal to 2.2 μm. The channel length of a transistor 190 inthe pixel portion is greater than or equal to 2.5 μm, or greater than orequal to 2.5 μm and less than or equal to 20 μm.

When the channel length of the transistor in the driver circuit portionis less than 2.5 μm or greater than or equal to 1.45 μm and less than orequal to 2.2 μm, as compared with the transistor in the pixel portion,the field-effect mobility can be increased, and the amount of on-statecurrent can be increased. As a result, a driver circuit portion that canoperate at high speed can be formed.

When the transistor in the driver circuit portion has high field-effectmobility, the number of input terminals can be made small.

FIG. 30 illustrates an example in which the transistor illustrated inFIG. 26A is used as the transistor in the pixel portion in the liquidcrystal display device in FIG. 2, and the transistor illustrated in FIG.29A is used as the transistor in the driver circuit portion. FIG. 31illustrates an example in which the transistor in the pixel portion andthe transistor in the driver circuit portion have different structuresin the EL display device in FIG. 3. Note that as the transistor in thepixel portion, any of the transistors illustrated in FIG. 26B and FIGS.27A and 27B can be used. As the transistor in the driver circuitportion, any of the transistors in which an oxide semiconductor layerhas a multi-layer structure in FIGS. 29A and 29B and FIGS. 27A and 27Bcan be used.

For the transistor in the pixel portion, a transistor with highreliability for light irradiation from the backlight or an EL element ispreferable. For example, an oxide semiconductor layer deposited by asputtering method using a material with an atomic ratio In:Ga:Zn=1:1:1as a target is used for a channel formation region, whereby a transistorwith high reliability for light irradiation can be formed.

In contrast, for the transistor in the driver circuit portion, atransistor with high field-effect mobility is preferable. Besides theabove structure, an oxide semiconductor layer deposited by a sputteringmethod using a material with an atomic ratio In:Ga:Zn=3:1:2 as a targetis used for a channel formation region, whereby a transistor with highfield-effect mobility can be formed.

In this embodiment, a method for forming the above two types oftransistors over one substrate is described with reference to FIGS. 32Ato 32D and FIGS. 33A to 33D. When one of the transistors has an oxidesemiconductor layer with a stacked structure, the two types oftransistors can be formed over one substrate with simple process. On theleft side of the drawing, a cross section in the channel lengthdirection of a transistor A whose structure is similar to that of thetransistor in FIG. 26A is shown, as the transistor in the pixel portion.On the right side of the drawing, a cross section in the channel lengthdirection of a transistor B whose structure is similar to that of thetransistor in FIG. 29A is shown, as the transistor in the driver circuitportion. Note that the reference numerals common in the transistor A andthe transistor B are given in only one of the transistors. In the methodfor forming the transistor described in this embodiment, a method forforming components (such as an organic resin layer) that are transferredto a flexible substrate, which is described in Embodiment 1, isincluded.

As the substrate 900, a glass substrate, a ceramic substrate, a quartzsubstrate, a sapphire substrate, or the like can be used. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like, acompound semiconductor substrate made of silicon germanium or the like,a silicon-on-insulator (SOI) substrate, or the like may be used. Stillalternatively, any of these substrates further provided with asemiconductor element may be used.

For the organic resin layer 910, an organic resin such as an epoxyresin, an aramid resin, an acrylic resin, a polyimide resin, a polyamideresin, or a polyamide-imide resin can be used. Above all, the polyimideresin is preferably used because it has high heat resistance. When thepolyimide resin is used, the thickness of the polyimide resin is greaterthan or equal to 3 nm and less than or equal to 20 μm, preferably,greater than or equal to 500 nm and less than or equal to 2 μm. Thepolyimide resin can be formed by a spin coating method, a dip coatingmethod, a doctor blade method, or the like.

The insulating film 915 can be formed using a silicon oxide film, asilicon oxynitride film, a silicon nitride film, a silicon nitride oxidefilm, or the like by a sputtering method, a CVD method, or the like.

An insulating layer 935 can be formed using an oxide insulating filmincluding aluminum oxide, magnesium oxide, silicon oxide, siliconoxynitride, gallium oxide, germanium oxide, yttrium oxide, zirconiumoxide, lanthanum oxide, neodymium oxide, hafnium oxide, tantalum oxide,or the like, a nitride insulating film including silicon nitride,silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or thelike, or a mixed material of any of these. Alternatively, a stackincluding any of the above materials may be used, and at least an upperlayer which is in contact with the oxide semiconductor layer ispreferably formed using a material containing excess oxygen that mightserve as a supply source of oxygen to the oxide semiconductor layer.

Oxygen may be added to the insulating layer 935 by an ion implantationmethod, an ion doping method, a plasma immersion ion implantationmethod, plasma treatment, or the like. Adding oxygen enables theinsulating layer 935 to supply oxygen much easily to the oxidesemiconductor layer.

In the case where a surface of the substrate 900 is made of an insulatorand there is no influence of impurity diffusion to the oxidesemiconductor layer to be formed later, the insulating layer 935 is notnecessarily provided. Furthermore, as illustrated in FIGS. 28A and 28B,the conductive film 921 is formed over the insulating film 915, and theinsulating layer 935 may be formed over the conductive film.

Next, over the insulating layer 935, a first oxide semiconductor film940 c that is to be a first oxide semiconductor layer 942 c and a secondoxide semiconductor film 940 b that is to be a second oxidesemiconductor layer 942 b in driver circuit transistors are deposited bya sputtering method, a CVD method, an MBE method, or the like.

Next, a resist mask 801 is formed in a driver circuit region using aresist mask 801 by a lithography method (see FIG. 32A). With use of theresist mask, the first oxide semiconductor film 940 c and the secondoxide semiconductor film 940 b are selectively etched, a stacked layerincluding the first oxide semiconductor layer 942 c and the second oxidesemiconductor layer 942 b is formed (see FIG. 32B).

Next, a third oxide semiconductor film 940 a that is to be a third oxidesemiconductor layer 942 a is formed to cover the stacked layer.

The materials described in Embodiment 5 can be used for the first oxidesemiconductor film 940 c, the second oxide semiconductor film 940 b, andthe third oxide semiconductor film 940 a. In this embodiment, forexample, an In—Ga—Zn-oxide (In:Ga:Zn=1:1:1 [atomic ratio]) is used forthe first oxide semiconductor film 940 c and the third oxidesemiconductor film 940 a, and an In—Ga—Zn-oxide (In:Ga:Zn=3:1:2 [atomicratio]) is used for the second oxide semiconductor film 940 b. Theproportion of each atom in the atomic ratio of the first oxidesemiconductor film 940 c, the second oxide semiconductor film 940 b, andthe third oxide semiconductor film 940 a may vary within a range of ±20%as an error. In the case where a sputtering method is used fordeposition, the above material can be used as a target.

An oxide semiconductor that can be used for each of the first oxidesemiconductor film 940 c, the second oxide semiconductor film 940 b, andthe third oxide semiconductor film 940 a preferably contains at leastindium (In) or zinc (Zn). Alternatively, both In and Zn are preferablycontained. In order to reduce fluctuations in electrical characteristicsof the transistors including the oxide semiconductor, the oxidesemiconductor preferably contains a stabilizer in addition to In and Zn.

As a stabilizer, gallium (Ga), tin (Sn), hafnium (Hf), aluminum (Al),zirconium (Zr), and the like can be given. As another stabilizer,lanthanoid such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), or lutetium (Lu) can be given.

As the oxide semiconductor, for example, any of the following can beused:

indium oxide, tin oxide, zinc oxide, an In—Zn oxide, a Sn—Zn oxide, anAl—Zn oxide, a Zn—Mg oxide, a Sn—Mg oxide, an In—Mg oxide, an In—Gaoxide, an In—Ga—Zn oxide, an In—Al—Zn oxide, an In—Sn—Zn oxide, aSn—Ga—Zn oxide, an Al—Ga—Zn oxide, a Sn—Al—Zn oxide, an In—Hf—Zn oxide,an In—La—Zn oxide, an In—Ce—Zn oxide, an In—Pr—Zn oxide, an In—Nd—Znoxide, an In—Sm—Zn oxide, an In—Eu—Zn oxide, an In—Gd—Zn oxide, anIn—Tb—Zn oxide, an In—Dy—Zn oxide, an In—Ho—Zn oxide, an In—Er—Zn oxide,an In—Tm—Zn oxide, an In—Yb—Zn oxide, an In—Lu—Zn oxide, an In—Sn-Ga—Znoxide, an In—Hf-Ga—Zn oxide, an In—Al-Ga—Zn oxide, an In—Sn—Al—Zn oxide,an In—Sn—Hf—Zn oxide, or an In—Hf—Al—Zn oxide.

Note that here, for example, an “In—Ga—Zn oxide” means an oxidecontaining In, Ga, and Zn as its main components. The In—Ga—Z-basedoxide may contain another metal element in addition to In, Ga, and Zn.Further, in this specification, a film formed using an In—Ga—Zn oxide isalso referred to as an IGZO film.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0 issatisfied, and m is not an integer) may be used. Note that M representsone or more metal elements selected from Ga, Y, Zr, La, Ce, and Nd.Alternatively, a material represented by In₂SnO₅(ZnO)_(n) (n>0, n is aninteger) may be used.

Note that a material of the second oxide semiconductor film 940 b isselected so that the second oxide semiconductor film 940 b has anelectron affinity higher than that of the first oxide semiconductor film940 c and that of the third oxide semiconductor film 940 a.

Note that the oxide semiconductor film is preferably formed by asputtering method. As a sputtering method, an RF sputtering method, a DCsputtering method, an AC sputtering method, or the like can be used. Toimprove uniformity of film thickness of the oxide semiconductor film,film composition, and crystallinity, a DC sputtering method or an ACsputtering method is preferably used rather than an RF sputteringmethod.

The indium content in the second oxide semiconductor film 940 b ispreferably higher than those in the first and third oxide semiconductorfilms 940 c and 940 a. In an oxide semiconductor, the s orbital of heavymetal mainly contributes to carrier transfer, and when the proportion ofIn in the oxide semiconductor is increased, overlap of the s orbitals islikely to be increased. Therefore, an oxide having a composition inwhich the proportion of In is higher than that of Ga has higher mobilitythan an oxide having a composition in which the proportion of In isequal to or lower than that of Ga. For this reason, with use of an oxidehaving a high indium content for the channel formation region, atransistor having high mobility can be achieved.

First heat treatment may be performed after the third oxidesemiconductor film 940 a is formed. The first heat treatment may beperformed at a temperature higher than or equal to 250° C. and lowerthan or equal to 650° C., preferably higher than or equal to 300° C. andlower than or equal to 500° C., in an inert gas atmosphere, anatmosphere containing an oxidizing gas at 10 ppm or more, or a reducedpressure state. Alternatively, the first heat treatment may be performedin such a manner that heat treatment is performed in an inert gasatmosphere, and then another heat treatment is performed in anatmosphere containing an oxidizing gas at 10 ppm or more, in order tocompensate desorbed oxygen. The first heat treatment can increase thecrystallinity of the first to third oxide semiconductor films 940 c to940 a and remove impurities such as water and hydrogen from the first tothird oxide semiconductor films 940 c to 940 a and the insulating layer935. Note that the first heat treatment may be performed after anetching step of the third oxide semiconductor film 940 a describedbelow.

Next, a resist mask 802 is formed in a pixel region by a lithographymethod. A resist mask 803 is formed over a stacked layer including thefirst oxide semiconductor layer 942 c and the second oxide semiconductorlayer 942 b in the driver circuit region (see FIG. 32C).

Next, with use of the resist mask, the third oxide semiconductor film940 a is selectively etched to form an oxide semiconductor layer 943 ain a pixel region. In addition, a stacked layer including the firstoxide semiconductor layer 942 c, the second oxide semiconductor layer942 b, and the third oxide semiconductor layer 942 a is formed in thedriver circuit region (see FIG. 32D).

Next, a first conductive film is formed over the oxide semiconductorlayer 943 a and the above stacked layer. As the first conductive film, asingle layer or a stacked layer can be formed using a material selectedfrom Al, Cr, Cu, Ta, Ti, Mo, W, Ni, Mn, Nd, and Sc and alloys of any ofthese metal materials.

Next, a resist mask is formed over the first conductive film, and withuse of the resist mask, the first conductive film is selectively etched,so that the source electrode layer 950 and the drain electrode layer 960are formed (see FIG. 33A). In this step, the oxide semiconductor layer943 a and part of the stacked layer including the first to third oxidesemiconductor layers becomes n-type.

Next, the gate insulating film 930 is formed to cover the pixel regionand the driver circuit region (see FIG. 33B). The gate insulating film930 can be formed using aluminum oxide, magnesium oxide, silicon oxide,silicon oxynitride, silicon nitride oxide, silicon nitride, galliumoxide, germanium oxide, yttrium oxide, zirconium oxide, lanthanum oxide,neodymium oxide, hafnium oxide, tantalum oxide, or the like. The gateinsulating film 930 may be a stack including any of the above materials.The gate insulating film 930 can be formed by a sputtering method, a CVDmethod, an MBE method, or the like.

Then, a second conductive film to be the gate electrode layer 920 isformed over the gate insulating film 930. For the second conductivefilm, a single layer, a stack, or an alloy of any of Al, Ti, Cr, Co, Ni,Cu, Y, Zr, Mo, Ru, Ag, Mn, Nd, Sc, Ta, and W can be used. The secondconductive film can be formed by a sputtering method, a CVD method, orthe like. The second conductive film may be formed using a conductivefilm containing nitrogen or a stack including the conductive film and aconductive film containing nitrogen.

After that, a resist mask is formed over the second conductive film, andthe second conductive film is selectively etched using the resist maskto form the gate electrode layer 920.

Next, an impurity 810 is added to regions that are not covered with thesource electrode layer 950, the drain electrode layer 960, and the gateelectrode layer 920, in the oxide semiconductor layer 943 a and thestacked layer including the first to third oxide semiconductor layers942 c to 942 a, so that the regions are made to be n-type regions andthe source region 951 and the drain region 961 are formed (see FIG.33C).

As a method for adding the impurity, plasma treatment, an ionimplantation method, an ion doping method, a plasma immersion ionimplantation method, or the like can be used. Note that the addition ofthe impurity may be performed after the gate insulating film 930 isselectively etched using the gate electrode layer 920 as a mask.

As an impurity added to increase the conductivity of an oxidesemiconductor layer, for example, one or more of the following can beused: phosphorus, arsenic, antimony, boron, aluminum, silicon, nitrogen,helium, neon, argon, krypton, xenon, indium, fluorine, chlorine,titanium, zinc, and carbon.

When a rare gas is added as an impurity element to the oxidesemiconductor layer, a bond between a metal element and oxygen in theoxide semiconductor layer is cut, whereby an oxygen vacancy isgenerated. Interaction between an oxygen vacancy in the oxidesemiconductor layer and hydrogen that remains in the oxide semiconductorlayer or is added to the oxide semiconductor layer later can increasethe conductivity of the oxide semiconductor layer. Specifically,hydrogen enters into the oxygen vacancies in the oxide semiconductorlayer, whereby an electron serving as a carrier is produced. As aresult, the conductivity is increased.

In FIG. 33C, when the width of a so-called offset region (a region inthe oxide semiconductor layer, which does not overlap with the gateelectrode layer 920, and the source and drain electrode layers) is lessthan 0.1 μm, doping with the above impurity is not necessarilyperformed. In the case where the offset region is less than 0.1 μm, adifference of the amount of off-state current of the transistor whetherthe doping with the impurity is performed or not is significantly small.

Next, over the gate insulating film 930 and a gate electrode layer 920,the insulating film 970, the insulating film 980, and the insulatingfilm 990 are formed (see FIG. 33D).

Oxygen may be added to the insulating film 970 and/or the insulatingfilm 980 by an ion implantation method, an ion doping method, a plasmaimmersion ion implantation method, plasma treatment, or the like. Byadding oxygen, oxygen can be easily supplied from the insulating film970 and/or the insulating film 980 to the oxide semiconductor layer 943a and the first to third oxide semiconductor layers 942 c to 942 a.

After that, second heat treatment may be performed. The second heattreatment can be performed in a condition similar to that of the firstheat treatment. By the second heat treatment, excess oxygen is easilyreleased from the insulating layer 935, the insulating film 970, and theinsulating film 980, and oxygen vacancies in the oxide semiconductorlayer 943 a and the stacked layer including the first to third oxidesemiconductor layers 942 c to 942 a can be reduced.

Furthermore, FIG. 34A to 34D illustrate a method for forming thefollowing structure: as a transistor in the pixel portion, a transistorC whose structure is similar to that of the transistor in FIG. 26B isused, and as a transistor in the driver circuit portion, a transistor Dwhose structure is similar to that of the transistor in FIG. 29B isused.

Up to the step illustrated in FIG. 33B, the steps similar to those inthe above method for forming the transistor described above areperformed, whereby the gate electrode layer 920 is formed (see FIG.34A).

Next, the gate insulating film 930 is etched using the gate electrodelayer 920 as a mask (see FIG. 34B).

Next, the insulating film 975 containing hydrogen, such as a siliconnitride film or an aluminum nitride film, is formed to be in contactwith part of the oxide semiconductor layer 940, so that hydrogen isdiffused to the part of the oxide semiconductor layer 940 (see FIG.34C). The diffused hydrogen is bonded to the oxygen vacancy in the oxidesemiconductor layer 940 and serves as a donor; accordingly, thelow-resistance source region 951 and the low-resistance drain region 961can be formed. In the structure in FIG. 34C, the oxide semiconductorlayer may be doped with the above impurity.

Next, over the insulating film 975, the insulating film 970, theinsulating film 980, and the insulating film 990 are formed (see FIG.34D).

Through the above steps, the transistor including an oxide semiconductorlayer with a single-layer structure and the transistor including anoxide semiconductor layer with a stacked structure can be easily formedover one substrate. In addition, a display device which can operate athigh speed, less deteriorates due to light irradiation, and includes apixel portion with excellent display quality can be formed.

Although the variety of films such as the metal films, the semiconductorfilms, and the inorganic insulating films which are described in thisembodiment typically can be formed by a sputtering method or a plasmaCVD method, such films may be formed by another method, e.g., a thermalCVD method. A metal organic chemical vapor deposition (MOCVD) method oran atomic layer deposition (ALD) method may be employed as an example ofa thermal CVD method.

A thermal CVD method has an advantage that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by a thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to the chamber at a time,the pressure in a chamber is set to an atmospheric pressure or a reducedpressure, and reaction is caused in the vicinity of the substrate orover the substrate.

Deposition by an ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching respective switching valves (also referred toas high-speed valves). For example, a first source gas is introduced, aninert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetition times of the sequenceof the gas introduction; therefore, an ALD method makes it possible toaccurately adjust a thickness and thus is suitable for manufacturing aminute FET.

The variety of films such as the metal film, the semiconductor film, andthe inorganic insulating film which have been disclosed in theembodiments can be formed by a thermal CVD method such as a MOCVD methodor an ALD method. For example, in the case where an In—Ga—Zn—O_(x) (x>0)film is formed, trimethylindium, trimethylgallium, and dimethylzinc canbe used. Note that the chemical formula of trimethylindium is In(CH₃)₃.The chemical formula of trimethylgallium is Ga(CH₃)₃. The chemicalformula of dimethylzinc is Zn(CH₃)₂. Without limitation to the abovecombination, triethylgallium (chemical formula: Ga(C₂H₅)₃) can be usedinstead of trimethylgallium and diethylzinc (chemical formula:Zn(C₂H₅)₂) can be used instead of dimethylzinc.

For example, in the case where a hafnium oxide film is formed by adeposition apparatus using an ALD method, two kinds of gases, i.e.,ozone (O₃) as an oxidizer and a source gas which is obtained byvaporizing liquid containing a solvent and a hafnium precursor compound(a hafnium alkoxide solution, typically tetrakis(dimethylamide)hafnium(TDMAH)) are used. Note that the chemical formula oftetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of anothermaterial liquid include tetrakis(ethylmethylamide)hafnium.

For example, in the case where an aluminum oxide film is formed using adeposition apparatus employing ALD, two kinds of gases, e.g., H₂O as anoxidizer and a source gas which is obtained by vaporizing a solvent andliquid containing an aluminum precursor compound (e.g.,trimethylaluminum (TMA)) are used. Note that the chemical formula oftrimethylaluminum is Al(CH₃)₃. Examples of another material liquidinclude tris(dimethylamide)aluminum, triisobutylaluminum, and aluminumtris(2,2,6,6-tetramethyl-3,5-heptanedionate).

For example, in the case where a silicon oxide film is formed by adeposition apparatus using an ALD method, hexachlorodisilane is adsorbedon a surface where a film is to be formed, chlorine contained in theadsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ ordinitrogen monoxide) are supplied to react with the adsorbate.

For example, in the case where a tungsten film is formed using adeposition apparatus employing ALD, a WF₆ gas and a B₂H₆ gas aresequentially introduced plural times to form an initial tungsten film,and then a WF₆ gas and an H₂ gas are introduced at a time, so that atungsten film is formed. Note that an SiH₄ gas may be used instead of aB₂H₆ gas.

For example, in the case where an oxide semiconductor film, e.g., anInGaZnO_(x) (x>0) film is formed using a deposition apparatus employingALD, an In(CH₃)₃ gas and an O₃ gas) are sequentially introduced pluraltimes to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas) areintroduced at a time to form a GaO layer, and then a Zn(CH₃)₂ gas and anO₃ gas) are introduced at a time to form a ZnO layer. Note that theorder of these layers is not limited to this example. A mixed compoundlayer such as an In—Ga—O layer, an In—Zn—O layer, or a Ga—Zn-O layer maybe formed by mixing of these gases. Note that although an H₂O gas whichis obtained by bubbling with an inert gas such as Ar may be used insteadof an O₃ gas), it is preferable to use an O₃ gas), which does notcontain H. Further, instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ gas may beused. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Further, aZn(CH₃)₂ gas may be used.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 7

In this embodiment, an oxide semiconductor film that can be used for atransistor according to one embodiment of the present invention isdescribed.

<Structure of Oxide Semiconductor>

The structure of an oxide semiconductor is described below.

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −10° andless than or equal to 10°, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.A term “substantially parallel” indicates that the angle formed betweentwo straight lines is greater than or equal to −30° and less than orequal to 30°. The term “perpendicular” indicates that the angle formedbetween two straight lines is greater than or equal to 80° and less thanor equal to 100°, and accordingly includes the case where the angle isgreater than or equal to 85° and less than or equal to 95°. A term“substantially perpendicular” indicates that the angle formed betweentwo straight lines is greater than or equal to 60° and less than orequal to 120°.

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a microcrystalline oxide semiconductor, and an amorphousoxide semiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and a microcrystalline oxide semiconductor.

<CAAC-OS>

First, a CAAC-OS is described. Note that a CAAC-OS can be referred to asan oxide semiconductor including c-axis aligned nanocrystals (CANC).

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, in the high-resolutionTEM image, a boundary between pellets, that is, a grain boundary is notclearly observed. Thus, in the CAAC-OS, a reduction in electron mobilitydue to the grain boundary is less likely to occur.

The CAAC-OS observed with a TEM is described below. FIG. 21A shows anexample of a high-resolution TEM image of a cross section of the CAAC-OSlayer which is observed from a direction substantially parallel to thesample surface. The high-resolution TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image. Note that theCs-corrected high-resolution TEM image can be obtained with, forexample, an atomic resolution analytical electron microscope JEM-ARM200Fmanufactured by JEOL Ltd.

FIG. 21B is an enlarged Cs-corrected high-resolution TEM image of aregion (1) in FIG. 21A. FIG. 21B shows that metal atoms are arranged ina layered manner in a pellet. Each metal atom layer has a configurationreflecting unevenness of a surface over which the CAAC-OS is formed(hereinafter, the surface is referred to as a formation surface) or atop surface of the CAAC-OS, and is arranged parallel to the formationsurface or the top surface of the CAAC-OS.

As shown in FIG. 21B, the CAAC-OS film has a characteristic atomicarrangement. The characteristic atomic arrangement is denoted by anauxiliary line in FIG. 21C. FIGS. 21B and 21C prove that the size of apellet is approximately 1 nm to 3 nm, and the size of a space caused bytilt of the pellets is approximately 0.8 nm. Therefore, the pellet canalso be referred to as a nanocrystal (nc).

Here, according to the Cs-corrected high-resolution TEM images, theschematic arrangement of pellets 5100 of a CAAC-OS layer over asubstrate 5120 is illustrated by such a structure in which bricks orblocks are stacked (see FIG. 21D). The part in which the pellets aretilted as observed in FIG. 21C corresponds to a region 5161 shown inFIG. 21D.

FIG. 22A shows a Cs-corrected high-resolution TEM image of a plane ofthe CAAC-OS observed from a direction substantially perpendicular to thesample surface. FIGS. 22B, 22C, and 22D are enlarged Cs-correctedhigh-resolution TEM images of regions (1), (2), and (3) in FIG. 22A,respectively. FIGS. 22B, 22C, and 22D indicate that metal atoms arearranged in a triangular, quadrangular, or hexagonal configuration in apellet. However, there is no regularity of arrangement of metal atomsbetween different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalis analyzed by an out-of-plane method, a peak appears at a diffractionangle (2θ) of around 31° as shown in FIG. 23A. This peak is derived fromthe (009) plane of the InGaZnO₄ crystal, which indicates that crystalsin the CAAC-OS have c-axis alignment, and that the c-axes are aligned ina direction substantially perpendicular to the formation surface or thetop surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-planemethod, another peak may appear when 2θ is around 36°, in addition tothe peak at 2θ of around 31°. The peak of 2θ at around 36° indicatesthat a crystal having no c-axis alignment is included in part of theCAAC-OS. It is preferable that in the CAAC-OS analyzed by anout-of-plane method, a peak appear when 2θ is around 31° and that a peaknot appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on a sample in a directionsubstantially perpendicular to the c-axis, a peak appears when 2θ isaround 56°. This peak is derived from the (110) plane of the InGaZnO₄crystal. In the case of the CAAC-OS, when analysis (ϕ scan) is performedwith 2θ fixed at around 56° and with the sample rotated using a normalvector of the sample surface as an axis (ϕ axis), as shown in FIG. 23B,a peak is not clearly observed. In contrast, in the case of a singlecrystal oxide semiconductor of InGaZnO₄, when ϕ scan is performed with2θ fixed at around 56°, as shown in FIG. 23C, six peaks which arederived from crystal planes equivalent to the (110) plane are observed.Accordingly, the structural analysis using XRD shows that the directionsof a-axes and b-axes are different in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS layer including an InGaZnO₄ crystal in a directionparallel to the sample surface, a diffraction pattern (also referred toas a selected-area transmission electron diffraction pattern) shown inFIG. 40A might be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 40B shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 40B, a ring-like diffraction pattern isobserved. Thus, the electron diffraction also indicates that the a-axesand b-axes of the pellets included in the CAAC-OS do not have regularalignment. It is supposed that the first ring in FIG. 40B is derivedfrom the (010) plane, the (100) plane, and the like of the crystals ofInGaZnO₄. The second ring in FIG. 40B is considered to be derived fromthe (110) plane and the like.

Moreover, the CAAC-OS is an oxide semiconductor having a low density ofdefect states. Defects in the oxide semiconductor are, for example, adefect due to impurity and oxygen vacancies. Thus, the CAAC-OS can bereferred to as an oxide semiconductor having a low impurityconcentration. Furthermore, the CAAC-OS can be referred to as an oxidesemiconductor with few oxygen vacancies.

The impurity contained in the oxide semiconductor might serve as acarrier trap or serve as a carrier generation source. Furthermore,oxygen vacancies in the oxide semiconductor serve as carrier traps orserve as carrier generation sources when hydrogen is captured therein.

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

An oxide semiconductor having a low density of defect states (a smallnumber of oxygen vacancies) can have a low carrier density. Such anoxide semiconductor is referred to as a highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductor. A CAAC-OShas a low impurity concentration and a low density of defect states.That is, a CAAC-OS is likely to be highly purified intrinsic orsubstantially highly purified intrinsic oxide semiconductors. Thus, atransistor including a CAAC-OS rarely has negative threshold voltage (israrely normally on). The highly purified intrinsic or substantiallyhighly purified intrinsic oxide semiconductor has few carrier traps. Anelectric charge trapped by the carrier traps in the oxide semiconductortakes a long time to be released. The trapped electric charge may behavelike a fixed electric charge. Thus, the transistor which includes theoxide semiconductor having a high impurity concentration and a highdensity of defect states might have unstable electrical characteristics.However, a transistor including a CAAC-OS has small variation inelectrical characteristics and high reliability.

Since the CAAC-OS has a low density of defect states, carriers generatedby light irradiation or the like are less likely to be trapped in defectstates. Therefore, in a transistor using the CAAC-OS, change inelectrical characteristics due to irradiation with visible light orultraviolet light is small.

<Microcrystalline Oxide Semiconductor>

Next, a microcrystalline oxide semiconductor is described.

A microcrystalline oxide semiconductor has a region in which a crystalpart is observed and a region in which a crystal part is not observedclearly in a high-resolution TEM image. In most cases, the size of acrystal part included in the microcrystalline oxide semiconductor isgreater than or equal to 1 nm and less than or equal to 100 nm, orgreater than or equal to 1 nm and less than or equal to 10 nm. An oxidesemiconductor including a nanocrystal that is a microcrystal with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, or a sizegreater than or equal to 1 nm and less than or equal to 3 nm isspecifically referred to as a nanocrystalline oxide semiconductor(nc-OS). In a high-resolution TEM image of the nc-OS, for example, agrain boundary is not clearly observed in some cases. Note that there isa possibility that the origin of the nanocrystal is the same as that ofa pellet in a CAAC-OS. Therefore, a crystal part of the nc-OS may bereferred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm, inparticular, a region with a size greater than or equal to 1 nm and lessthan or equal to 3 nm) has a periodic atomic arrangement. There is noregularity of crystal orientation between different pellets in thenc-OS. Thus, the orientation of the whole film is not observed.Accordingly, in some cases, the nc-OS cannot be distinguished from anamorphous oxide semiconductor, depending on an analysis method. Forexample, when the nc-OS is subjected to structural analysis by anout-of-plane method with an XRD apparatus using an X-ray having adiameter larger than the size of a pellet, a peak which shows a crystalplane does not appear. Furthermore, a diffraction pattern like a halopattern is observed when the nc-OS is subjected to electron diffractionusing an electron beam with a probe diameter (e.g., 50 nm or larger)that is larger than the size of a pellet (the electron diffraction isalso referred to as selected-area electron diffraction). Meanwhile,spots appear in a nanobeam electron diffraction pattern of the nc-OSwhen an electron beam having a probe diameter close to or smaller thanthe size of a pellet is applied. Moreover, in a nanobeam electrondiffraction pattern of the nc-OS, regions with high luminance in acircular (ring) pattern are shown in some cases. Also in a nanobeamelectron diffraction pattern of the nc-OS layer, a plurality of spots isshown in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

Thus, the nc-OS is an oxide semiconductor that has high regularity ascompared to an amorphous oxide semiconductor. Therefore, the nc-OS islikely to have a lower density of defect states than an amorphous oxidesemiconductor. Note that there is no regularity of crystal orientationbetween different pellets in the nc-OS. Therefore, the nc-OS has ahigher density of defect states than the CAAC-OS.

<Amorphous Oxide Semiconductor>

Next, an amorphous oxide semiconductor is described.

The amorphous oxide semiconductor is such an oxide semiconductor havingdisordered atomic arrangement and no crystal part. For example, theamorphous oxide semiconductor does not have a specific state as inquartz.

In a high-resolution TEM image of the amorphous oxide semiconductor,crystal parts cannot be found.

When the amorphous oxide semiconductor is subjected to structuralanalysis by an out-of-plane method with an XRD apparatus, a peak whichshows a crystal plane does not appear. A halo pattern is observed whenthe amorphous oxide semiconductor is subjected to electron diffraction.Furthermore, a spot is not observed and only a halo pattern appears whenthe amorphous oxide semiconductor is subjected to nanobeam electrondiffraction.

There are various understandings of an amorphous structure. For example,a structure whose atomic arrangement does not have ordering at all iscalled a completely amorphous structure. Meanwhile, a structure whichhas ordering until the nearest neighbor atomic distance or thesecond-nearest neighbor atomic distance but does not have long-rangeordering is also called an amorphous structure. Therefore, the strictestdefinition does not permit an oxide semiconductor to be called anamorphous oxide semiconductor as long as even a negligible degree ofordering is present in an atomic arrangement. At least an oxidesemiconductor having long-term ordering cannot be called an amorphousoxide semiconductor. Accordingly, because of the presence of a crystalpart, for example, a CAAC-OS and an nc-OS cannot be called an amorphousoxide semiconductor or a completely amorphous oxide semiconductor.

<Amorphous-Like Oxide Semiconductor Layer>

Note that an oxide semiconductor may have a structure between the nc-OSand the amorphous oxide semiconductor. The oxide semiconductor havingsuch a structure is specifically referred to as an amorphous-like oxidesemiconductor (a-like OS).

In a high-resolution TEM image of the a-like OS, a void may be observed.Furthermore, in the high-resolution TEM image, there are a region wherea crystal part is clearly observed and a region where a crystal part isnot observed.

The a-like OS has an unstable structure because it includes a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS (Sample A), an nc-OS (Sample B), and a CAAC-OS (Sample C)are prepared as samples subjected to electron irradiation. Each of thesamples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

Note that which part is regarded as a crystal part is determined asfollows. It is known that a unit cell of the InGaZnO₄ crystal has astructure in which nine layers including three In—O layers and sixGa—Zn—O layers are stacked in the c-axis direction. The distance betweenthe adjacent layers is equivalent to the lattice spacing on the (009)plane (also referred to as d value). The value is calculated to be 0.29nm from crystal structural analysis. Accordingly, a portion where thelattice spacing between lattice fringes is greater than or equal to 0.28nm and less than or equal to 0.30 nm is regarded as a crystal part ofInGaZnO₄. Each of lattice fringes corresponds to the a-b plane of theInGaZnO₄ crystal.

FIG. 41 shows the change in the average size of crystal parts (at 22points to 45 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 41 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose. Specifically, as shown by (1) in FIG. 41, acrystal part of approximately 1.2 nm at the start of TEM observation(the crystal part is also referred to as an initial nucleus) grows to asize of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by(2) and (3) in FIG. 41, the average crystal sizes in an nc-OS layer anda CAAC-OS layer are approximately 1.4 nm and approximately 2.1 nm,respectively, regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit includes a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor film.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that single crystals with the same composition do not exist in somecases. In that case, single crystal oxide semiconductors with differentcompositions are combined at an adequate ratio, which makes it possibleto calculate density equivalent to that of a single crystal oxidesemiconductor with the desired composition. The density of a singlecrystal oxide semiconductor having the desired composition can becalculated using a weighted average according to the combination ratioof the single crystal oxide semiconductors with different compositions.Note that it is preferable to use as few kinds of single crystal oxidesemiconductors as possible to calculate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedfilm including two or more films of an amorphous oxide semiconductor, ana-like OS, a microcrystalline oxide semiconductor, and a CAAC-OS, forexample.

<Deposition Model>

Examples of deposition models of a CAAC-OS and an nc-OS are describedbelow.

FIG. 42A is a schematic view of the inside of a deposition chamber wherea CAAC-OS film is deposited by a sputtering method.

A target 5130 is attached to a backing plate. A plurality of magnets areprovided to face the target 5130 with the backing plate positionedtherebetween. The plurality of magnets generate a magnetic field. Asputtering method in which the disposition speed is increased byutilizing a magnetic field of magnets is referred to as a magnetronsputtering method.

A substrate 5120 is placed to face the target 5130, and the distance d(also referred to as a target-substrate distance (T-S distance)) isgreater than or equal to 0.01 m and less than or equal to 1 m,preferably greater than or equal to 0.02 m and less than or equal to 0.5m. The deposition chamber is mostly filled with a deposition gas (e.g.,an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol %or higher) and the pressure in the deposition chamber is controlled tobe higher than or equal to 0.01 Pa and lower than or equal to 100 Pa,preferably higher than or equal to 0.1 Pa and lower than or equal to 10Pa. Here, discharge starts by application of a voltage at a constantvalue or higher to the target 5130, and plasma is observed. The magneticfield forms a high-density plasma region in the vicinity of the target5130. In the high-density plasma region, the deposition gas is ionized,so that an ion 5101 is generated. Examples of the ion 5101 include anoxygen cation (O⁺) and an argon cation (Ar⁺).

Here, the target 5130 has a polycrystalline structure which includes aplurality of crystal grains and in which a cleavage plane exists in atleast one crystal grain. FIG. 43A shows a structure of an InGaZnO₄crystal included in the target 5130 as an example. Note that FIG. 43Ashows a structure of the case where the InGaZnO₄ crystal is observedfrom a direction parallel to the b-axis. FIG. 43A indicates that oxygenatoms in a Ga—Zn—O layer are positioned close to those in an adjacentGa—Zn—O layer. The oxygen atoms have negative charge, whereby repulsiveforce is generated between the two Ga—Zn—O layers. As a result, theInGaZnO₄ crystal has a cleavage plane between the two adjacent Ga—Zn—Olayers.

The ion 5101 generated in the high-density plasma region is acceleratedtoward the target 5130 side by an electric field, and then collides withthe target 5130. At this time, a pellet 5100 a and a pellet 5100 b whichare flat-plate-like (pellet-like) sputtered particles are separated andsputtered from the cleavage plane. Note that structures of the pellet5100 a and the pellet 5100 b may be distorted by an impact of collisionof the ion 5101.

The pellet 5100 a is a flat-plate-like (pellet-like) sputtered particlehaving a triangle plane, e.g., regular triangle plane. The pellet 5100 bis a flat-plate-like (pellet-like) sputtered particle having a hexagonplane, e.g., regular hexagon plane. Note that flat-plate-like(pellet-like) sputtered particles such as the pellet 5100 a and thepellet 5100 b are collectively called pellets 5100. The shape of a flatplane of the pellet 5100 is not limited to a triangle or a hexagon. Forexample, the flat plane may have a shape formed by combining two or moretriangles. For example, a quadrangle (e.g., rhombus) may be formed bycombining two triangles (e.g., regular triangles).

The thickness of the pellet 5100 is determined depending on the kind ofdeposition gas and the like. The thicknesses of the pellets 5100 arepreferably uniform; the reasons thereof are described later. Inaddition, the sputtered particle preferably has a pellet shape with asmall thickness as compared to a dice shape with a large thickness. Forexample, the thickness of the pellet 5100 is greater than or equal to0.4 nm and less than or equal to 1 nm, preferably greater than or equalto 0.6 nm and less than or equal to 0.8 nm. In addition, for example,the width of the pellet 5100 is greater than or equal to 1 nm and lessthan or equal to 3 nm, preferably greater than or equal to 1.2 nm andless than or equal to 2.5 nm. The pellet 5100 corresponds to the initialnucleus in the description of (1) in FIG. 41. For example, when the ion5101 collides with the target 5130 including an In—Ga—Zn oxide, thepellet 5100 that includes three layers of a Ga—Zn—O layer, an In—Olayer, and a Ga—Zn—O layer as shown in FIG. 43B is separated. Note thatFIG. 43C shows the structure of the separated pellet 5100 which isobserved from a direction parallel to the c-axis. Therefore, the pellet5100 has a nanometer-sized sandwich structure including two Ga—Zn—Olayers (pieces of bread) and an In—O layer (filling).

The pellet 5100 may receive a charge when passing through the plasma, sothat side surfaces thereof are negatively or positively charged. In thepellet 5100, an oxygen atom positioned on its side surface may benegatively charged. In this manner, when the side surfaces are chargedwith the same polarity, charges repel each other, and accordingly, thepellet 5100 can maintain a flat-plate shape. In the case where a CAAC-OSis an In—Ga—Zn oxide, there is a possibility that an oxygen atom bondedto an indium atom is negatively charged. There is another possibilitythat an oxygen atom bonded to an indium atom, a gallium atom, or a zincatom is negatively charged. In addition, the pellet 5100 may grow bybeing bonded with an indium atom, a gallium atom, a zinc atom, an oxygenatom, or the like when passing through plasma. A difference in sizebetween (2) and (1) in FIG. 41 corresponds to the amount of growth inplasma. Here, in the case where the temperature of the substrate 5120 isat around room temperature, the pellet 5100 on the substrate 5120 hardlygrows; thus, an nc-OS film is formed (see FIG. 42B). An nc-OS can bedeposited when the substrate 5120 has a large size because thedeposition of an nc-OS can be carried out at room temperature. Note thatin order that the pellet 5100 grows in plasma, it is effective toincrease deposition power in sputtering. High deposition power canstabilize the structure of the pellet 5100.

As shown in FIGS. 42A and 42B, the pellet 5100 flies like a kite inplasma and flutters up to the substrate 5120. Since the pellets 5100 arecharged, when the pellet 5100 gets close to a region where anotherpellet 5100 has already been deposited, repulsion is generated. Here,above the substrate 5120, a magnetic field in a direction parallel tothe top surface of the substrate 5120 (also referred to as a horizontalmagnetic field) is generated. A potential difference is given betweenthe substrate 5120 and the target 5130, and accordingly, current flowsfrom the substrate 5120 toward the target 5130. Thus, the pellet 5100 isgiven a force (Lorentz force) on the top surface of the substrate 5120by an effect of the magnetic field and the current. This is explainablewith Fleming's left-hand rule.

The mass of the pellet 5100 is larger than that of an atom. Therefore,to move the pellet 5100 over the top surface of the substrate 5120, itis important to apply some force to the pellet 5100 from the outside.One kind of the force may be force which is generated by the action of amagnetic field and current. In order to apply a sufficient force to thepellet 5100 so that the pellet 5100 moves over a top surface of thesubstrate 5120, it is preferable to provide, on the top surface, aregion where the magnetic field in a direction parallel to the topsurface of the substrate 5120 is 10 G or higher, preferably 20 G orhigher, further preferably 30 G or higher, still further preferably 50 Gor higher. Alternatively, it is preferable to provide, on the topsurface, a region where the magnetic field in a direction parallel tothe top surface of the substrate 5120 is 1.5 times or higher, preferablytwice or higher, further preferably 3 times or higher, still furtherpreferably 5 times or higher as high as the magnetic field in adirection perpendicular to the top surface of the substrate 5120.

At this time, the magnets and the substrate 5120 are moved or rotatedrelatively, whereby the direction of the horizontal magnetic field onthe top surface of the substrate 5120 continues to change. Therefore,the pellet 5100 can be moved in various directions on the top surface ofthe substrate 5120 by receiving forces in various directions.

Furthermore, as shown in FIG. 42A, when the substrate 5120 is heated,resistance between the pellet 5100 and the substrate 5120 due tofriction or the like is low. As a result, the pellet 5100 glides abovethe top surface of the substrate 5120. The glide of the pellet 5100 iscaused in a state where the flat plane faces the substrate 5120. Then,when the pellet 5100 reaches the side surface of another pellet 5100that has been already deposited, the side surfaces of the pellets 5100are bonded. At this time, the oxygen atom on the side surface of thepellet 5100 is released. With the released oxygen atom, oxygen vacanciesin a CAAC-OS is filled in some cases; thus, the CAAC-OS has a lowdensity of defect states. Note that the temperature of the top surfaceof the substrate 5120 is, for example, higher than or equal to 100° C.and lower than 500° C., higher than or equal to 150° C. and lower than450° C., or higher than or equal to 170° C. and lower than 400° C.Hence, even when the substrate 5120 has a large size, it is possible todeposit a CAAC-OS.

Furthermore, the pellet 5100 is heated on the substrate 5120, wherebyatoms are rearranged, and the structure distortion caused by thecollision of the ion 5101 can be reduced. The pellet 5100 whosestructure distortion is reduced is substantially single crystal. Evenwhen the pellets 5100 are heated after being bonded, expansion andcontraction of the pellet 5100 itself hardly occur, which is caused byturning the pellet 5100 into substantially single crystal. Thus,formation of defects such as a grain boundary due to expansion of aspace between the pellets 5100 can be prevented, and accordingly,generation of crevasses can be prevented.

The CAAC-OS does not have a structure like a board of a single crystaloxide semiconductor but has arrangement with a group of pellets 5100(nanocrystals) like stacked bricks or blocks. Furthermore, a grainboundary does not exist between the pellets 5100. Therefore, even whendeformation such as shrink occurs in the CAAC-OS owing to heating duringdeposition, heating or bending after deposition, it is possible torelieve local stress or release distortion. Therefore, this structure issuitable for a flexible semiconductor device. Note that the nc-OS hasarrangement in which pellets 5100 (nanocrystals) are randomly stacked.

When the target 5130 is sputtered with the ion 5101, in addition to thepellets 5100, zinc oxide or the like may be separated. The zinc oxide islighter than the pellet and thus reaches the top surface of thesubstrate 5120 before the pellet. As a result, the zinc oxide forms azinc oxide layer 5102 with a thickness greater than or equal to 0.1 nmand less than or equal to 10 nm, greater than or equal to 0.2 nm andless than or equal to 5 nm, or greater than or equal to 0.5 nm and lessthan or equal to 2 nm. FIGS. 44A to 44D are cross-sectional schematicviews.

As illustrated in FIG. 44A, a pellet 5105 a and a pellet 5105 b aredeposited over the zinc oxide layer 5102. Here, side surfaces of thepellet 5105 a and the pellet 5105 b are in contact with each other. Inaddition, a pellet 5105 c is deposited over the pellet 5105 b, and thenglides over the pellet 5105 b. Furthermore, a plurality of particles5103 separated from the target together with the zinc oxide iscrystallized by heating of the substrate 5120 to form a region 5105 a 1on another side surface of the pellet 5105 a. Note that the plurality ofparticles 5103 may contain oxygen, zinc, indium, gallium, or the like.

Then, as illustrated in FIG. 44B, the region 5105 a 1 grows to part ofthe pellet 5105 a to form a pellet 5105 a 2. In addition, a side surfaceof the pellet 5105 c is in contact with another side surface of thepellet 5105 b.

Next, as illustrated in FIG. 44C, a pellet 5105 d is deposited over thepellet 5105 a 2 and the pellet 5105 b, and then glides over the pellet5105 a 2 and the pellet 5105 b. Furthermore, a pellet 5105 e glidestoward another side surface of the pellet 5105 c over the zinc oxidelayer 5102.

Then, as illustrated in FIG. 44D, the pellet 5105 d is placed so that aside surface of the pellet 5105 d is in contact with a side surface ofthe pellet 5105 a 2. Furthermore, a side surface of the pellet 5105 e isin contact with another side surface of the pellet 5105 c. A pluralityof particles 5103 separated from the target 5130 together with the zincoxide is crystallized by heating of the substrate 5120 to form a region5105 d 1 on another side surface of the pellet 5105 d.

As described above, deposited pellets are placed to be in contact witheach other and then growth is caused at side surfaces of the pellets,whereby a CAAC-OS is formed over the substrate 5120. Therefore, eachpellet of the CAAC-OS is larger than that of the nc-OS. A difference insize between (3) and (2) in FIG. 41 corresponds to the amount of growthafter deposition.

When spaces between pellets are extremely small, the pellets may form alarge pellet. The large pellet has a single crystal structure. Forexample, the size of the pellet may be greater than or equal to 10 nmand less than or equal to 200 nm, greater than or equal to 15 nm andless than or equal to 100 nm, or greater than or equal to 20 nm and lessthan or equal to 50 nm, when seen from the above. In this case, in anoxide semiconductor used for a minute transistor, a channel formationregion might be fit inside the large pellet. Therefore, the regionhaving a single crystal structure can be used as the channel formationregion. Furthermore, when the size of the pellet is increased, theregion having a single crystal structure can be used as the channelformation region, the source region, and the drain region of thetransistor.

In this manner, when the channel formation region or the like of thetransistor is formed in a region having a single crystal structure, thefrequency characteristics of the transistor can be increased in somecases.

It is considered that as shown in such a model, the pellets 5100 aredeposited on the substrate 5120. Thus, a CAAC-OS can be deposited evenwhen a formation surface does not have a crystal structure; therefore, agrowth mechanism in this case is different from epitaxial growth. Inaddition, laser crystallization is not needed for formation of aCAAC-OS, and a uniform film can be formed even over a large-sized glasssubstrate or the like. For example, even when the top surface (formationsurface) of the substrate 5120 has an amorphous structure (e.g., the topsurface is formed of amorphous silicon oxide), a CAAC-OS can be formed.

In addition, it is found that in formation of the CAAC-OS, the pellets5100 are arranged in accordance with the top surface shape of thesubstrate 5120 that is the formation surface even when the formationsurface has unevenness. For example, in the case where the top surfaceof the substrate 5120 is flat at the atomic level, the pellets 5100 arearranged so that flat planes parallel to the a-b plane face downwards.In the case where the thicknesses of the pellets 5100 are uniform, alayer with a uniform thickness, flatness, and high crystallinity isformed. By stacking n layers (n is a natural number), the CAAC-OS can beobtained.

In the case where the top surface of the substrate 5120 has unevenness,a CAAC-OS in which n layers (n is a natural number) in each of which thepellets 5100 are arranged along the unevenness are stacked is formed.Since the substrate 5120 has unevenness, a gap is easily generatedbetween in the pellets 5100 in the CAAC-OS in some cases. Note thatowing to intermolecular force, the pellets 5100 are arranged so that agap between the pellets is as small as possible even on the unevennesssurface. Therefore, even when the formation surface has unevenness, aCAAC-OS with high crystallinity can be obtained.

Since the CAAC-OS film is deposited in accordance with such a model, thesputtered particle preferably has a pellet shape with a small thickness.Note that when the sputtered particles has a dice shape with a largethickness, planes facing the substrate 5120 vary, which may lead toformation of a film whose thickness or crystal alignment is notuniformed.

According to the deposition model described above, a CAAC-OS with highcrystallinity can be formed even on a film formation surface with anamorphous structure.

The structure described in this embodiment can be used in appropriatecombination with the structure described in any of the otherembodiments.

Embodiment 8

In this embodiment, electronic devices to which one embodiment of thepresent invention is applied will be described with reference to FIGS.24A to 24D.

Highly reliable flexible electronic devices can be manufactured byadopting the device of one embodiment of the present invention.

Examples of the electronic devices are a television device, a monitor ofa computer or the like, a camera such as a digital camera or a digitalvideo camera, a digital photo frame, a mobile phone (also referred to asa cellular phone or a cellular phone device), a portable game console, aportable information terminal, an audio reproducing device, alarge-sized game machine, and the like.

The display device of one embodiment of the present invention hasflexibility and thus can be incorporated along a curved inside/outsidewall surface of a house or a building or a curved interior/exteriorsurface of a car.

FIG. 24A illustrates an example of a mobile phone. A mobile phone 7100includes a display portion 7102 incorporated in a housing 7101,operation buttons 7103, an external connection port 7104, a speaker7105, a microphone 7106, a camera 7107, and the like. Note that themobile phone 7100 is manufactured using the display device of oneembodiment of the present invention for the display portion 7102.According to one embodiment of the present invention, a highly reliablemobile phone having a curved display portion can be provided.

When the display portion 7102 of the mobile phone 7100 illustrated inFIG. 24A is touched with a finger or the like, data can be input to themobile phone 7100. Operations such as making a call and entering acharacter can be performed by touch on the display portion 7102 with afinger or the like. For example, by touching an icon 7108 displayed onthe display portion 7102, application can be started.

The power can be turned on or off with the operation buttons 7103. Inaddition, types of images displayed on the display portion 7102 can beswitched; switching images from a mail creation screen to a main menuscreen.

FIG. 24B illustrates an example of a wrist-watch-type portableinformation terminal. A portable information terminal 7200 includes ahousing 7201, a display portion 7202, a band 7203, a buckle 7204, anoperation button 7205, an input output terminal 7206, and the like.

The portable information terminal 7200 is capable of executing a varietyof applications such as mobile phone calls, e-mailing, viewing andediting texts, music reproduction, Internet communication, and acomputer game.

The display surface of the display portion 7202 is bent, and images canbe displayed on the bent display surface. Further, the display portion7202 includes a touch sensor, and operation can be performed by touchingthe screen with a finger, a stylus, or the like. For example, bytouching an icon 7207 displayed on the display portion 7202, applicationcan be started.

With the operation button 7205, a variety of functions such as powerON/OFF, ON/OFF of wireless communication, setting and cancellation ofmanner mode, and setting and cancellation of power saving mode can beperformed. For example, the functions of the operation button 7205 canbe set freely by setting the operation system incorporated in theportable information terminal 7200.

The portable information terminal 7200 can employ near fieldcommunication that is a communication method in accordance with anexisting communication standard. In that case, for example, mutualcommunication between the portable information terminal 7200 and aheadset capable of wireless communication can be performed, and thushands-free calling is possible.

Moreover, the portable information terminal 7200 includes the inputoutput terminal 7206, and data can be directly transmitted to andreceived from another information terminal via a connector. Powercharging through the input output terminal 7206 is possible. Note thatthe charging operation may be performed by wireless power feedingwithout using the input output terminal 7206.

The display device of one embodiment of the present invention can beused in the display portion 7202 of the portable information terminal7200.

FIG. 24C illustrates an example of a portable display device. A displaydevice 7300 includes a housing 7301, a display portion 7302, operationbuttons 7303, a display portion pull 7304, and a control portion 7305.

The display device 7300 includes a rolled flexible display portion 7102in the cylindrical housing 7301.

The display device 7300 can receive a video signal with the controlportion 7305 and can display the received video on the display portion7302. In addition, a battery is included in the control portion 7305.Moreover, a terminal portion for connecting a connector may be includedin the control portion 7305 so that a video signal or power can bedirectly supplied from the outside with a wiring.

By pressing the operation buttons 7303, power ON/OFF, switching ofdisplayed videos, and the like can be performed.

FIG. 24D illustrates a display device 7300 in a state where the displayportion 7302 is pulled out with the display portion pull 7304. Videoscan be displayed on the display portion 7302 in this state. Further, theoperation buttons 7303 on the surface of the housing 7301 allowone-handed operation. The operation button 7303 is provided not in thecenter of the housing 7301 but on one side of the housing 7301 asillustrated in FIG. 24C, which makes one-handed operation easy.

Note that a reinforcement frame may be provided for a side portion ofthe display portion 7302 so that the display portion 7302 has a flatdisplay surface when pulled out.

Note that in addition to this structure, a speaker may be provided forthe housing so that sound is output with an audio signal receivedtogether with a video signal.

The display portion 7302 includes the light-emitting device of oneembodiment of the present invention. According to one embodiment of thepresent invention, a lightweight and highly reliable light-emittingdevice can be provided.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

Embodiment 9

In this embodiment, modification examples of the transistor described inthe above embodiment will be described with reference to FIGS. 37A to37F, FIGS. 38A to 38F, and FIGS. 39A to 39E. The transistors illustratedin FIGS. 37A to 37F each includes, over a substrate 821, an oxidesemiconductor layer 828 over an insulating film 824, an insulating film837 in contact with the oxide semiconductor layer 828, and a conductivefilm 840 in contact with the insulating film 837 and overlapping withthe oxide semiconductor layer 828. The insulating film 837 functions asa gate insulating film. The conductive film 840 functions as a gateelectrode layer.

In addition, the transistors are provided with an insulating film 846 incontact with the oxide semiconductor layer 828 and an insulating film847 in contact with the insulating film 846. Moreover, conductive films856 and 857 in contact with the oxide semiconductor layer 828 throughthe openings in the insulating film 846 and the insulating film 847 areprovided. The conductive films 856 and 857 function as a sourceelectrode layer and a drain electrode layer. Furthermore, an insulatingfilm 862 in contact with the insulating film 847 and the conductivefilms 856 and 857 is provided.

As the structures of the transistor described in this embodiment and theconductive film and the insulating film in contact with the structures,the structures of the transistor described in the above embodiment andthe conductive film and the insulating film in contact with thestructures can be used as appropriate.

In the transistor illustrated in FIG. 37A, the oxide semiconductor layer828 includes a region 828 a overlapping with the conductive film 840 andregions 828 b and 828 c containing an impurity element. The regions 828b and 828 c are formed so that the region 828 a is sandwichedtherebetween. The conductive films 856 and 857 are in contact with theregions 828 b and 828 c respectively. The region 828 a functions as achannel region. The regions 828 b and 828 c have lower resistivity thanthe region 828 a and can be referred to as low resistance regions. Theregions 828 b and 828 c function as a source region and a drain region.

Alternatively, as in the transistor illustrated in FIG. 37B, the oxidesemiconductor layer 828 may have a structure in which an impurityelement is not added to regions 828 d and 828 e in contact with theconductive films 856 and 857. In this case, the regions 828 b and 828 ccontaining an impurity element are provided between the region 828 a andthe regions 828 d and 828 e in contact with the conductive films 856 and857. The regions 828 d and 828 e have conductivity when the voltage isapplied to the conductive films 856 and 857; thus, the regions 828 d and828 e function as a source region and a drain region.

Note that the transistor illustrated in FIG. 37B can be formed in such amanner that after the conductive films 856 and 857 are formed, animpurity element is added to the oxide semiconductor layer using theconductive film 840 and the conductive films 856 and 857 as masks.

An end portion of the conductive film 840 may have a tapered shape. Theangle θ1 formed between a surface where the insulating film 837 and theconductive film 840 are in contact with each other and a side surface ofthe conductive film 840 may be less than 90°, greater than or equal to10° and less than or equal to 85°, greater than or equal to 15° and lessthan or equal to 85°, greater than or equal to 30° and less than orequal to 85°, greater than or equal to 45° and less than or equal to85°, or greater than or equal to 60° and less than or equal to 85°. Whenthe angle θ1 is less than 90°, greater than or equal to 10° and lessthan or equal to 85°, greater than or equal to 15° and less than orequal to 85°, greater than or equal to 30° and less than or equal to85°, greater than or equal to 45° and less than or equal to 85°, orgreater than or equal to 60° and less than or equal to 85°, the coverageof the side surfaces of the insulating film 837 and the conductive film840 with the insulating film 846 can be improved.

Next, modification examples of the regions 828 b and 828 c aredescribed. FIGS. 37C to 37F are each an enlarged view of the vicinity ofthe oxide semiconductor layer 828 illustrated in FIG. 37A. The channellength L indicates a distance between a pair of regions containing animpurity element.

As illustrated in FIG. 37C in a cross-sectional view in the channellength direction, the boundaries between the region 828 a and theregions 828 b and 828 c are aligned or substantially aligned with theend portion of the conductive film 840 with the insulating film 837interposed therebetween. In other words, the boundaries between theregion 828 a and the regions 828 b and 828 c are aligned orsubstantially aligned with the end portion of the conductive film 840,seen in a top view.

Alternatively, as illustrated in FIG. 37D in a cross-sectional view inthe channel length direction, the region 828 a has a region that doesnot overlap with the end portion of the conductive film 840. The regionfunctions as an offset region. The length of the offset region in thechannel length direction is referred to as L_(off). Note that when aplurality of offset regions are provided, L_(off) indicates the lengthof one offset region. L_(off) is included in the channel length L. Notethat L_(off) is smaller than 20%, smaller than 10%, smaller than 5%, orsmaller than 2% of the channel length L.

Alternatively, as illustrated in FIG. 37E in a cross-sectional view inthe channel length direction, the regions 828 b and 828 c each have aregion overlapping with the conductive film 840 with the insulating film837 interposed therebetween. The regions function as an overlap region.The overlap region in the channel length direction is referred to asL_(ov). L_(ov) is smaller than 20%, smaller than 10%, smaller than 5%,or smaller than 2% of the channel length L.

Alternatively, as illustrated in FIG. 37F in a cross-sectional view inthe channel length direction, a region 828 f is provided between theregion 828 a and the region 828 b, and a region 828 g is providedbetween the region 828 a and the region 828 c. The regions 828 f and 828g have lower concentration of an impurity element and higher resistivitythan the regions 828 b and 828 c. Although the regions 828 f and 828 goverlap with the insulating film 837 in this case, they may overlap withthe insulating film 837 and the conductive film 840.

Note that in FIGS. 37C to 37F, the transistor illustrated in FIG. 37A isdescribed; however, the transistor illustrated in FIG. 37B can employany of the structures in FIGS. 37C to 37F as appropriate.

In the transistor illustrated in FIG. 38A, the end portion of theinsulating film 837 is positioned on an outer side than the end portionof the conductive film 840. In other words, the insulating film 837 hasa shape such that the end portion extends beyond the end portion of theconductive film 840. The insulating film 846 can be kept away from theregion 828 a; thus, nitrogen, hydrogen, and the like contained in theinsulating film 846 can be prevented from entering the region 828 afunctioning as a channel region.

In the transistor illustrated in FIG. 38B, the insulating film 837 andthe conductive film 840 each have a tapered shape, and the angles of thetapered shapes are different from each other. In other words, the angleθ1 formed between a surface where the insulating film 837 and theconductive film 840 are in contact with each other and a side surface ofthe conductive film 840 is different from an angle θ2 formed between asurface where the oxide semiconductor layer 828 and the insulating film837 are in contact with each other and the side surface of theinsulating film 837. The angle θ2 may be less than 90°, greater than orequal to 30° and less than or equal to 85°, or greater than or equal to45° and less than or equal to 70°. For example, when the angle θ2 isless than the angle θ1, the coverage with the insulating film 846 isimproved. Alternatively, when the angle θ2 is greater than the angle θ1,the insulating film 846 can be kept away from the region 828 a; thus,nitrogen, hydrogen, or the like contained in the insulating film 846 canbe prevented from entering the region 828 a functioning as a channelregion.

Next, modification examples of the regions 828 b and 828 c are describedwith reference to FIGS. 38C to 38F. Note that FIGS. 38C to 38F are eachan enlarged view of the vicinity of the oxide semiconductor layer 828illustrated in FIG. 38A.

As illustrated in FIG. 38C in a cross-sectional view in the channellength direction, the boundaries between the region 828 a and theregions 828 b and 828 c are aligned or substantially aligned with theend portion of the conductive film 840 with the insulating film 837interposed therebetween. In other words, seen in the top view, theboundaries between the region 828 a and the regions 828 b and 828 c arealigned or substantially aligned with the end portion of the conductivefilm 840.

As illustrated in FIG. 38D in a cross-sectional view in the channellength direction, the region 828 a has a region that does not overlapwith the conductive film 840. The region functions as an offset region.In other words, seen in the top view, the end portions of the regions828 b and 828 c are aligned or substantially aligned with the endportion of the insulating film 837 and do not overlap with the endportion of the conductive film 840.

As illustrated in FIG. 38E in a cross-sectional view in the channellength direction, the regions 828 b and 828 c each have a regionoverlapping with the conductive film 840 with the insulating film 837interposed therebetween. Such a region is referred to as an overlapregion. In other words, seen in the top view, the end portions of theregions 828 b and 828 c overlap with the conductive film 840.

As illustrated in FIG. 38F in a cross-sectional view in the channellength direction, the region 828 f is provided between the region 828 aand the region 828 b, and the region 828 g is provided between theregion 828 a and the region 828 c. The regions 828 f and 828 g havelower concentration of an impurity element and higher resistivity thanthe regions 828 b and 828 c. Although the regions 828 f and 828 goverlap with the insulating film 837 in this case, they may overlap withthe insulating film 837 and the conductive film 840.

Note that in FIGS. 38C to 38F, the transistor illustrated in FIG. 38A isdescribed; however, the transistor illustrated in FIG. 38B can employany of the structures in FIGS. 38C to 38F as appropriate.

In the transistor illustrated in FIG. 39A, the conductive film 840 has astacked structure including a conductive film 840 a in contact with theinsulating film 837 and a conductive film 840 b in contact with theconductive film 840 a. The end portion of the conductive film 840 a ispositioned on an outer side than the end portion of the conductive film840 b. In other words, the conductive film 840 a has such a shape thatthe end portion extends beyond the end portion of the conductive film840 b.

Next, modification examples of the regions 828 b and 828 c aredescribed. Note that FIGS. 39B to 39E are each an enlarged view in thevicinity of the oxide semiconductor layer 828 illustrated in FIG. 39A.

As illustrated in FIG. 39B in a cross-sectional view in the channellength direction, the boundaries between the region 828 a and theregions 828 b and 828 c are aligned or substantially aligned with theend portion of the conductive film 840 a in the conductive film 840 withthe insulating film 837 interposed therebetween. In other words, seen inthe top view, the boundaries between the region 828 a and the regions828 b and 828 c are aligned or substantially aligned with the endportion of the conductive film 840.

As illustrated in FIG. 39C in a cross-sectional view in the channellength direction, the region 828 a has a region that does not overlapwith the conductive film 840. The region functions as an offset region.The structure may have such a structure that, seen in the top view, theend portions of the regions 828 b and 828 c are aligned or substantiallyaligned with the end portion of the insulating film 837 and do notoverlap with the end portion of the conductive film 840.

As illustrated in FIG. 39D in a cross-sectional view in the channellength direction, the regions 828 b and 828 c each have a regionoverlapping with the conductive film 840, specifically the conductivefilm 840 a. Such a region is referred to as an overlap region. In otherwords, seen in a top view, the end portions of the regions 828 b and 828c overlap with the conductive film 840 a.

As illustrated in FIG. 39E in a cross-sectional view in the channellength direction, the region 828 f is provided between the region 828 aand the region 828 b, and the region 828 g is provided between theregion 828 a and the region 828 c. The impurity element is added to theregions 828 f and 828 g through the conductive film 840 a; thus, theregions 828 f and 828 g have lower concentration of impurity element andhigher resistivity than the regions 828 b and 828 c. Although theregions 828 f and 828 g overlap with the conductive film 840 a, they mayoverlap with both the conductive film 840 a and the conductive film 840b.

The end portion of the insulating film 837 may be positioned on theouter side than the end portion of the conductive film 840 a.

Alternatively, the side surface of the insulating film 837 may becurved.

Alternatively, the insulating film 837 may have a tapered shape. Inother words, an angle formed between a surface where the oxidesemiconductor layer 828 and the insulating film 837 are in contact witheach other and a side surface of the insulating film 837 may be lessthan 90°, preferably greater than or equal to 30° and less than 90°.

As described with FIGS. 39A to 39E, the oxide semiconductor layer 828includes the regions 828 f and 828 g having lower concentration of animpurity element and higher resistivity than the regions 828 b and 828c, whereby the electric field of the drain region can be relaxed. Thus,a deterioration of the transistor due to the electric field of the drainregion, such as a shift of the threshold voltage of the transistor, canbe inhibited.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

EXPLANATION OF REFERENCE

101: structure body, 101 a: rotator, 101 b: member, 102: trigger, 103:processed member, 103 a: member, 103 b: member, 104: portion, 105:stage, 107: guide, 108: arrow, 109: rotation axis, 111: member, 151:structure body, 152: structure body, 153: processed member, 153 a:member, 153 b: member, 155: stage, 156: stage, 157: support, 158:transfer roller, 159: rotation axis, 160: gate insulating film, 161:member, 162: trigger, 170: gate electrode layer, 190: transistor, 194:transistor, 300: display device, 300 a: display device, 300 b: displaydevice, 301: flexible substrate, 302: pixel portion, 304: circuitportion, 305: circuit portion, 307: flexible substrate, 308: FPCterminal portion, 310: signal line, 311: wiring portion, 312: sealant,316: FPC, 318 a: bonding layer, 318 b: bonding layer, 320 a: organicresin layer, 320 b: organic resin layer, 321 a: insulating film, 321 b:insulating film, 334: insulating film, 336: coloring layer, 338:light-blocking layer, 350: transistor, 352: transistor, 360: connectionelectrode, 364: insulating film, 366: insulating film, 368: insulatingfilm, 370: planarization insulating film, 372: conductive film, 374:conductive film, 375: liquid crystal element, 376: liquid crystal layer,378: spacer, 380: anisotropic conductive film, 400: display device, 408:FPC, 410: element layer, 411: element layer, 430: insulating film, 432:sealing layer, 434: insulating film, 444: conductive film, 446: ELlayer, 448: conductive film, 462: substrate, 463: substrate, 468:ultraviolet light, 480: light-emitting element, 501: pixel circuit, 502:pixel portion, 504: driver circuit portion, 504 a: gate driver, 504 b:source driver, 506: protection circuit, 507: terminal portion, 550:transistor, 552: transistor, 554: transistor, 560: capacitor, 562:capacitor, 570: liquid crystal element, 572: light-emitting element,600: excimer laser device, 610 a: laser light, 610 b: laser light, 610c: laser light, 610 d: linear beam, 630: optical system, 650: mirror,670: lens, 700: processing object, 710: processing region, 720:substrate, 801: resist mask, 802: resist mask, 803: resist mask, 810:impurity, 821: substrate, 824: insulating film, 828: oxide semiconductorlayer, 828 a: region, 828 b: region, 828 c: region, 828 d: region, 828e: region, 828 f: region, 828 g: region, 828 h: region, 828 i: region,837: insulating film, 840: conductive film, 840 a: conductive film, 840b: conductive film, 846: insulating film, 847: insulating film, 856:conductive film, 857: conductive film, 862: insulating film, 900:substrate, 910: organic resin layer, 915: insulating film, 920: gateelectrode layer, 921: conductive film, 930: gate insulating film, 931:insulating film, 932: insulating film, 933: insulating film, 935:insulating layer, 940: oxide semiconductor layer, 940 a: oxidesemiconductor film, 940 b: oxide semiconductor film, 940 c: oxidesemiconductor film, 941 a: oxide semiconductor layer, 941 b: oxidesemiconductor layer, 942 a: oxide semiconductor layer, 942 b: oxidesemiconductor layer, 942 c: oxide semiconductor layer, 943 a: oxidesemiconductor layer, 950: source electrode layer, 951: source region,960: drain electrode layer, 961: drain region, 970: insulating film,975: insulating film, 980: insulating film, 990: insulating film, 7100:mobile phone, 7101: housing, 7102: display portion, 7103: operationbutton, 7104: external connection port, 7105: speaker, 7106: microphone,7107: camera, 7108: icon, 7200: portable information terminal, 7201:housing, 7202: display portion, 7203: band, 7204: buckle, 7205:operation button, 7206: input output terminal, 7207: icon, 7300: displaydevice, 7301: housing, 7302: display portion, 7303: operation button,7304: display portion pull, 7305: control portion, 5100: pellet, 5100 a:pellet, 5100 b: pellet, 5101: ion, 5102: zinc oxide layer, 5103:particle, 5105 a: pellet, 5105 a 1: region, 5105 a 2: pellet, 5105 b:pellet, 5105 c: pellet, 5105 d: pellet, 5105 d 1: region, 5105 e:pellet, 5120: substrate, 5130: target, 5161: region, 8000: displaymodule, 8001: upper cover, 8002: lower cover, 8003: FPC, 8004: touchpanel, 8005: FPC, 8006: display panel, 8007: backlight unit, 8008: lightsource, 8009: frame, 8010: printed board, 8011: battery

This application is based on Japanese Patent Application serial no.2013-249631 filed with Japan Patent Office on Dec. 2, 2013, JapanesePatent Application serial no. 2013-256872 filed with Japan Patent Officeon Dec. 12, 2013, Japanese Patent Application serial no. 2013-272176filed with Japan Patent Office on Dec. 27, 2013, and Japanese PatentApplication serial no. 2014-047348 filed with Japan Patent Office onMar. 11, 2014, the entire contents of which are hereby incorporated byreference.

What is claimed is:
 1. An electronic device comprising: a first flexiblesubstrate; a first bonding layer over the first flexible substrate; afirst organic resin layer over the first bonding layer; an insulatinglayer over the first organic resin layer; a transistor over theinsulating layer; an organic electroluminescent element electricallyconnected to the transistor; a second organic resin layer over theorganic electroluminescent element; a second bonding layer over thesecond organic resin layer; and a second flexible substrate over thesecond bonding layer, wherein a thickness of the first organic resinlayer is less than or equal to 20 and wherein at least a part of theelectronic device is curved.
 2. The electronic device according to claim1, wherein the first organic resin layer includes at least one of anepoxy resin, an aramid resin, an acrylic resin, a polyimide resin, apolyamide resin, and a polyamide-imide resin.
 3. The electronic deviceaccording to claim 1, wherein the first bonding layer includes at leastone of an epoxy resin, an acrylic resin, a silicone resin, and a phenolresin.
 4. The electronic device according to claim 1, further comprisinga color filter over the organic electroluminescent element.
 5. Theelectronic device according to claim 1, wherein the electronic device isa portable information terminal.
 6. An electronic device comprising: afirst flexible substrate; a first bonding layer over the first flexiblesubstrate; a first organic resin layer over the first bonding layer; aninsulating layer over the first organic resin layer; a transistor overthe insulating layer; an organic electroluminescent element electricallyconnected to the transistor; a touch sensor over the organicelectroluminescent element; a second organic resin layer over theorganic electroluminescent element; a second bonding layer over thesecond organic resin layer; and a second flexible substrate over thesecond bonding layer, wherein a thickness of the first organic resinlayer is less than or equal to 20 □m, wherein at least a part of theelectronic device is curved, and wherein the electronic device ischarged by wireless power feeding.
 7. The electronic device according toclaim 6, wherein the first organic resin layer includes at least one ofan epoxy resin, an aramid resin, an acrylic resin, a polyimide resin, apolyamide resin, and a polyamide-imide resin.
 8. The electronic deviceaccording to claim 6, wherein the first bonding layer includes at leastone of an epoxy resin, an acrylic resin, a silicone resin, and a phenolresin.
 9. The electronic device according to claim 6, further comprisinga color filter over the organic electroluminescent element.
 10. Theelectronic device according to claim 6, wherein the electronic device isa portable information terminal.
 11. An electronic device comprising: afirst flexible substrate; a first bonding layer over the first flexiblesubstrate; a first organic resin layer over the first bonding layer; aninsulating layer over the first organic resin layer; a transistor overthe insulating layer; an organic electroluminescent element electricallyconnected to the transistor; a second organic resin layer over theorganic electroluminescent element; a second bonding layer over thesecond organic resin layer; and a second flexible substrate over thesecond bonding layer, wherein a thickness of the first organic resinlayer is less than or equal to 20 □m, wherein at least a part of theelectronic device is curved, and wherein the transistor includes anoxide semiconductor layer.
 12. The electronic device according to claim11, wherein the oxide semiconductor layer includes indium and zinc. 13.The electronic device according to claim 11, wherein the first organicresin layer includes at least one of an epoxy resin, an aramid resin, anacrylic resin, a polyimide resin, a polyamide resin, and apolyamide-imide resin.
 14. The electronic device according to claim 11,wherein the first bonding layer includes at least one of an epoxy resin,an acrylic resin, a silicone resin, and a phenol resin.
 15. Theelectronic device according to claim 11, further comprising a colorfilter over the organic electroluminescent element.
 16. The electronicdevice according to claim 11, wherein the electronic device is aportable information terminal.